Doubtless the Germans could come up with some figures proving that the iS’s various body addenda, including the comical rear diffuser, add crucial stability to the X5 at autobahn speeds, but the effect on the high street is less certain. The 20-inch wheels are so wide the thing appears to roll along on four beer barrels.
All X5s look tougher up front following the recent restyle, when they gained a recontoured bonnet and more aggressive grille, but the iS adds further menace with a reshaped front valance. There are two iS-only paint treatments: this Imola red and Le Mans blue, as seen on M3s everywhere, and two chromed rectangular exhaust tips poking into the face of following traffic. But if you want the showy aluminium running boards, or the swivelling headlights, your bill will get bigger.
There’s more to the most expensive X5 than simple cosmetics, however. The success of Porsche’s rapid Cayenne Turbo has proved that there is a small but affluent band of people willing to pay a large premium to sit high and travel very quickly. And in the absence of a proper M-power version, this is as hot a factory-produced X5 as you’ll get. M-badge or not, the figures don’t put it far off.
When BMW breathed on the 4.4-litre V8 at the time of last year’s facelift, power jumped from 286bhp to 320bhp, turning the standard car into quite a hotrod in the process. The 340bhp of the 4.6 wasn’t going to cut it this time around.
Stretching the excellent alloy V8 to 4.8 litres has provided the answer, endowing the iS with near-M5 levels of grunt: 380bhp at 6200rpm and 369lb ft at 3600rpm. BMW’s figures say 62mph arrives in just 6.1sec, which almost certainly means high fives to our 60mph yardstick – blistering pace by any standards, but quite ridiculous in something that weighs over two tonnes and is tall enough to let you lord it over van drivers.
That lofty driving position conspires with the sublime auto ’box, which slurs effortlessly between its six ratios, to rob you of some of the sensation of speed. And while it will eventually nudge 153mph, the cliff-like Cd factor means the rate of thrust isn’t quite so staggering beyond three figures. But few cars are likely to cause much of a problem to this X5 on the road. And with that angry snout looming in the mirrors, they’re likely to move over quick-smart anyway.
Yet it’s the pseudo-NASCAR growl from the fat rear pipes rather than the speed that leaves the most lasting impression. Blipping the throttle unleashes such a deliciously naughty cacophony of sound that you half expect it to settle back into a lumpy, kidney-haemorrhaging 500rpm idle, like the old Yanks it apes.
Thankfully the handling benchmark wasn’t a lump of ’60’s Detroit iron. Retuned springs and dampers tighten up body control but the steering is hugely heavy at low speeds and seems slightly duller than on lower models, perhaps due to the bigger footprint. Pedalling it quickly is fun, and most un-off-roader-like, but while scaring the life out of hot-hatch drivers soon becomes a favourite pastime, it’s not exactly an M5 on stilts.
We’ve criticised the ride of big-wheeled X5s before, but in the wake of the latest run-flat tyre-shod Fives, this one doesn’t seem so bad. Certainly it’s firm, particularly around town, and obviously less compliant than small-wheeled non-Sport-spec X5s, but it’s still bearable.
In the cabin it’s the details rather than the whole that let you know this is beyond X5: the M-style rev-counter lights that extinguish one by one as the engine oil warms; the oil-temperature gauge that replaces the pointless fuel-consumption meter within the speedo and a smattering of metal-look trim.
Unless you specify Nappa leather and Alcantara, that’s about your lot. And we continue to find it astonishing that BMW can charge nearly £60,000 for a car and still ask you to fork out more for a CD changer. The cabin is still great, though: roomy and beautifully constructed, simple but elegant and soon to be the last reminder of a different era of BMW design.
It’s hard to make a rational case for the 4.8iS, but then to do so would be to miss the point. It’s for people who want not necessarily the best, as the 3.0d is clearly a more rounded car, but the supreme X5. And when cost is no object and logic vanishes, the iS is that car.