Engine: 5.2-litre V10
Maximum power: 450PS at 7,000rpm
Maximum power: 540Nm at 3,500rpm
Transmission: All-wheel drive with six-speed "tiptronic" gearbox
Performance: 155 mph maximum (governed); 0 to 50mph in 3.1 secs; 0 to 62mph in 5.1 sec; 20.9mpg
At first glance, it's hard to justify the existence of the Audi S8. It's one of those "Audis with a Lamborghini engine", the 5.2-litre V10 as fitted to the Gallardo. It is a magical unit, for sure; almost as sonorous here as in its original home in that wedge of Italian style. Yet did we really need it in an Audi barge?
In reality, I note, the Lamborghini engine owes more than a little to Audi, for Audi is the loving mother of Lamborghini these days. She's just taken back from her offspring what was originally hers. And it's that remarkable engine that both makes this car hard to justify and easy to love.
First, and we may as well be frank about this, the S8 is no mate of Gaia. The official fuel consumption figures suggest that it's capable of 20mpg but, thanks to one of the Audi's many optional dashboard readouts, I'm confident in saying that my average was about half that.
This would suggest that the actual CO2 emissions are also adrift of the official numbers. According to the Government, this near-two-tonne machine will pump out 320 grams of carbon dioxide for every kilometre travelled; bad, but the reality is probably even worse.
All this, it has to be said, is true even though Audi's A8 range, of which the S8 is the sporting flagship, uses lots of weight-saving technology such as a space frame, aluminium body panels and an aluminium engine (although I guess the quattro all-wheel-drive transmission might be heavier than it ought to be).
So I would hope that the average buyer of this expensive motor car (£73,000 as tested) would limit excursions in it and spend most of their time in a Smart or in one of those electric G-Wiz cars, or in a little Citroën diesel, or - well, pretty much anything, because pretty much anything is better than an Audi S8 on pollution. You should also be prepared to keep your S8 for a very long time, because the longer you keep a car away from the scrapyard, the better it is for the world.
Now, I know that all this is supposed to be beside the point when it comes to a performance machine like the S8 - but why should it be? If we're serious about climate change, we should at least give a second thought to the planet when it comes to our motoring choices. If you don't accept the scientific consensus, fine, don't worry. Otherwise, you do have to wonder about pottering around at 11.9mpg.
Still, when it comes to durability - which is an important point in assessing greenness - this Audi looks as though it's ready to stay around for a long, long time.
As with every A8 I've driven, it is superbly well constructed. Even a few years into its life-span the interior still looks contemporary, especially when the woodwork that usually adorns the cabin is replaced by some carbon fibre-effect trim. The seats adjust every which way; there's a four-zone climate control; and the leather is dead comfy. Our S8 was even fitted with a TV, an £850 option (my six-year-old co-driver found it too low to see properly, but you can't win 'em all).
There's a colour camera that shows you where you're reversing; the boot has an electric motor so it opens and closes itself; and - what else? Ah yes, the heated seats have six warmth settings to choose from, and there are two illuminated interior mirrors, one at normal magnification and one that's electron-microscope strong for getting a real close-up of those spots. Handy.
Still, you can get all that on an A8 with any of the usual VW/Audi group engines, including their muscular V8 and W12 petrol units. Why, then, go for 10 cylinders when eight or 12 already do such a magnificent job of moving this Audi around? How can they justify yet another engine option?
You won't know until you drive it, but I'll try to explain. You see, the V10 doesn't necessarily go that much faster than the V8 or the W12, and all A8s are governed to do no more than 155mph anyway. It's how they do their job that's the point. The V8 and the W12 do things without undue fuss; the V10 doesn't really give a damn.
The Audi S8, in other words, inherits some of its Lamborghini half-brother's sense of urgency. You have to get used to a very frisky machine. You have to get used to the accelerator's extreme alacrity: not enough and you'll kangaroo along, too much and you will flatten yourself and your passengers against the seat backs. It's fun, but it's not really what you're supposed to do in a limo.
The real laughs, though, begin when you investigate the paddle shifts. Usually, I disregard these because they never seem to add that much to the driving experience, but I tried them early on in my time with the S8. The car was transformed. In normal D mode, it's fast; and the S sports mode is more rapid still, with more of that metallic growl emanating from up front. Play around with the paddle shifts and suddenly you're in a great big Lamborghini with four doors and an Audi badge. It's like administering mind-altering drugs to some sober-sided German businessperson. That, alone, is enough to justify the S8's existence.
Jaguar XJ/R £62,040
To some, the XJ is classically beautiful; to others, it's fuddy-duddy. Still, this version is one rude Jag; a proper V8 and a snug cabin, and more reliable these days.
Maserati Quattroporte Sport GT £80,595
Don't overlook this, a new version of an old(ish) favourite that doesn't actually go any faster. The gearchange is its biggest flaw.
Mercedes-Benz CLS 63 AMG £72,995
Smaller and sexier than its competitors here, the AMG treatment results in a very hard-edged machine. Nimble, and the V8 is unfailingly delightful.