With the number of premium-brand models proliferating practically exponentially, buying a luxury vehicle might not seem as exclusive as it once did. No worries about that with the 2015 BMW Alpina B7, though. This customized version of BMW's full-size 7 Series sedan starts life on the BMW assembly line and is then modified by German performance specialist Alpina. It's so uncommon that it's barely known outside the BMW inner circle. You're highly unlikely ever to see another B7 in your town, let alone in your neighborhood.

The sheer exclusivity of the 2015 BMW Alpina B7 might be its most prominent calling card, but Alpina's performance upgrades and upscale but deliciously discreet exterior and interior modifications are the tangible aspects that put the B7 at the top of BMW's 7 Series lineup. Its thunderous turbocharged V8 engine has numerous internal enhancements to boost it to 540 horsepower, which is nearly 100 hp stronger than the same engine in BMW's standard 750i. In addition, Alpina's careful retuning of this big sedan's suspension is a brilliant fusion of control and comfort.

Inside, Alpina fits the 2015 B7 with unique wood trim and a lavishly leathered steering wheel, as well as special sill plates and other subtle detailing such as an instrument cluster with Alpina-specific graphics. Expanding the B7's rarity factor are the almost limitless personalization options available from Alpina and BMW's Individual program. Heck, you can choose exactly how thick you want the B7's steering wheel, though as you might imagine, digging deep into the customization catalogs will get expensive.

Like the production-line BMW 7 Series, the 2015 Alpina B7 is available in standard- and long-wheelbase layouts; the standard wheelbase brings ample stretch-out room for both front and rear occupants, while the long-wheelbase B7 takes space to an almost decadent level, particularly in the rear. Either B7 model also is available with all-wheel drive in place of the standard rear-wheel drive. Downsides? The only meaningful pitfall is a shortage of interior storage space and a trunk that's a little skimpy, both surprising shortcomings in such a large sedan.

The 2015 BMW Alpina B6 isn't the only full-size luxury sedan dialing up the exclusivity ante, though. Audi offers the sport-tuned S8 version of its standard A8 large car – and although the S8 can't be personalized to the degree of the Alpina B7, it's also definitely less expensive. Mercedes-Benz has the titanic S65 AMG, a high-performance version of its storied S-Class full-size sedan that certainly makes its own elite statement. And for those who enjoy extracting the most performance from this class of super sedan, Porsche's Panamera Turbo is prodigiously fast, while few other automakers have the wealth of personalization options Porsche presents, including the Panamera's extremely limited-edition new Exclusive Series models. Despite the immense levels of performance and luxury that come with all these models, none present you with quite the same aura of customized cachet and rarity the Alpina connection imparts on the 2015 BMW Alpina B7.

The 2015 BMW Alpina B7 is a performance-themed version of the BMW 7 Series full-size luxury sedan (reviewed separately). It is available in both standard- and long-wheelbase models, with rear- or all-wheel drive available for either variation.

Standard features include 21-inch wheels with summer performance tires, LED headlights with automatic high-beam control, LED foglights, automatic windshield wipers, a power trunk lid, front and rear parking sensors, heated exterior mirrors, keyless ignition and entry, a sunroof, a heated steering wheel, a power rear sunshade, manual rear side sunshades, auto-dimming mirrors, four-zone automatic climate control, premium leather upholstery, 14-way heated and ventilated power front seats, driver memory settings, heated rear seats and wood interior trim.

Among the standard technology features in every Alpina B7 are a head-up display, a rearview camera, the BMW iDrive electronics interface, voice control, a navigation system, Bluetooth phone and audio connectivity, smartphone app integration, BMW Assist emergency communications and a 10-speaker Harman Kardon surround-sound audio system with a six-disc CD/DVD changer, satellite radio and iPod and USB input jacks.

The Alpina B7 offers a few stand-alone options and option packages. The Driver Assistance Plus package adds side and top camera views, lane-departure and blind-spot warning systems and a frontal-collision warning and mitigation system with pedestrian detection.

Choose either of the long-wheelbase models and the optional Luxury Rear Seating package configures the rear seating to two rear power-adjustable and ventilated rear seats with massage functions; this package reduces the car's total seating capacity to four. A rear-seat entertainment package adds two color screens to the front seatbacks, which can be operated independently and controlled by a rear iDrive controller. Single options for the 2015 BMW Alpina B7 include full Merino leather in a variety of colors, adaptive cruise control (bundled with frontal collision mitigation and pedestrian detection), a 16-speaker Bang & Olufsen surround-sound audio system and a night vision system with infrared pedestrian detection.

The 2015 BMW Alpina B7 is powered by a twin-turbocharged 4.4-liter V8 engine that produces 540 hp and 538 pound-feet of torque. An eight-speed automatic is the only transmission available and it includes manual shift control via steering-wheel-mounted buttons. In standard form, the B7 comes in a rear-wheel-drive configuration, but all-wheel drive (also known as xDrive) is available.

The EPA estimates fuel economy at 19 mpg combined (16 city/25 highway) for rear-wheel-drive models, while AWD versions deliver the same combined fuel economy rating but drops 1 mpg off the highway rating.

Standard safety features on the Alpina B7 include antilock brakes, traction and stability control, front and rear parking sensors, front side airbags, full-length side curtain airbags, front knee airbags and active front head restraints. Also standard is the BMW Assist emergency communications system, which includes automatic crash notification, an emergency response button, remote door unlock and stolen vehicle recovery.

Multiple parking cameras, adaptive cruise control, a blind-spot warning system, a lane-departure warning system, a night vision system and a frontal collision warning and mitigation system (which can detect both cars and pedestrians and apply the brakes to help you avoid a collision) are available.

It's difficult to choose what's more satisfying about the 2015 BMW Alpina B7's performance: its power or its handling. Although 540 hp pumping through twin turbochargers is enough to hurl the B7 from zero to 60 mph in 4.6 seconds, according to BMW, the bigger everyday delight might be this big sedan's precisely controlled yet completely compliant suspension, an area in which Alpina's modifiers take particular pride.

On many luxury sedans with large 21-inch wheels, low-profile tires and a tuned-up suspension, the ride is thumpy or brittle. Not so on the 2015 B7, for which no road irregularity seems to cause an abrupt disturbance or over-aggressive response. Yes, this car's ride is firm and cornering is handled in a no-nonsense fashion with almost no discomfiting body lean, but the overall ride never seems to suffer from this noticeably upgraded suspension performance.

If that's not enough, the suspension, throttle and transmission behavior can be adjusted by the driver to varying degrees of aggressiveness or comfort orientation. Some vehicles with similar setups display scant differences between the settings, but for the B7, each setting has a distinct effect on the car's dynamic behavior. Even the most performance-intent drivers may find the sportiest setting too harsh, particularly for its abrupt gearshifts and hyperactive throttle response.

Analogies between luxury cars and private jets are common, but seldom are they closer to the truth than with the 2015 Alpina B7. Throughout the interior, occupants are treated to rich materials everywhere they look and touch, along with equivalent comfort. The "multicontour" front seats are a particular treat: They can be adjusted to fit the body the way a decades-old catcher's mitt fits a baseball. And the active function for the front seats gently adds or reduces support according to the car's cornering demeanor.

Napa leather of noticeable richness is standard for the seats, but also covers the dashboard, a luxury touch that, along with unique-to-Alpina wood trim, reinforces this car's hand-crafted feel, while the full suite of BMW's well-designed and well-executed electronics provide all the technology expected of a vehicle in this class, including an immensely useful head-up display that can project on the windshield your next move from the navigation system and the always-improving iDrive interface for controlling almost all in-vehicle functions.

Nobody in the rear of the 2015 Alpina B7 is ever going to feel cheated either. There's a glorious amount of legroom and headroom, with the long-wheelbase model adding a decadent 6 inches of extra legroom. Don't anticipate ever needing to seat three back there? The optional Luxury Rear Seating package once again brings to mind the private-jet reference with two separated rear seats that can heat, cool and massage.

For a car this large, some might wonder why there's a comparative dearth of handy interior storage areas. Unless you use the smallish center storage box, it's hard to find somewhere to secure even a cell phone. And it seems there should be more than 14.0 cubic feet of trunk space in such a grandly proportioned sedan with the kind of superlative comfort that tempts long-distance interstate cruising.