Nobody needs a 513bhp, 5.15-metre limousine capable of lunging from 0-62mph in 4.2secs. For a start, that sort of acceleration makes it hard for your average company director to focus on that morning's FT, pinned as they would be to the leather upholstery of the S8's rear seat. And besides, have you seen the pump-price of fuel? Surely every sensible chauffeur-driven chap makes sure his motor glides along on a pillow of economic turbodiesel efficiency?
Fortunately not. Sure, the vast majority of modern luxobarges do run on the black stuff, and there's nothing wrong with that. But for the chap (or chappess) who wants mighty performance from their executive express, and hang the logic of it, there are still a few choices: the 503bhp Jaguar XJ Supersport, the 536bhp Mercedes S63 AMG, or this, the 513bhp Audi S8.
But the S8 also sings for its supper in economical terms. Despite its prodigious pace, the fastest A8 manages a creditable 27.7mpg on the EU combined cycle. It does so courtesy of a bag of tricks that includes the conventional (a lowest-in-class drag coefficient of 0.28) and the innovative (as well as stop-start, the new 4.0-litre twin-turbo V8 now has cylinder de-activation technology). It's clever stuff, this shutting down of cylinders: depending on how consistently or gently you're driving, and as long as you're running between 960 and 3500rpm, the S8 becomes a four-cylinder car. And Audi reckons this is worth a 10 per cent improvement in fuel economy during normal, everyday driving.
The sharper PHers out there might also have noticed that we keep talking about V8s and turbos; for the new S8, the old V10 has been replaced by an all-new 4.0-litre twin-turbo motor. And very impressive it is. Apart from those stonking power figures, the new engine is also light (at 220kg) and compact (it's just 497mm long) and many of the ancillaries, including the exhaust manifold, turbos and intercooler, are nestled within the vee of the engine.
On twisty roads the S8 does a pretty impressive job of hiding its bulk. The sport differential metes power out to the rear wheels as you accelerate out of corners, and the quick steering helps it to feel nimble. It's pretty impressive stuff. The optional ceramic brakes can get overworked, though. They stop you well enough, but you can gather sufficient speed that prolonged, repeated heavy braking can cause more than a whiff of smoke to rise from the wheelarches - though even then you won't find any fade. But this is a two-tonne car; the laws of physics can be stretched, but they can't be beaten.
The Audi S8 is a fine bit of kit, then: it's fast, it's clever and it handles pretty well. And we haven't yet mentioned the truly lovely, cosseting interior. Or the price: the Audi starts from below £80k, whereas a Merc S63 AMG won't leave you change from £112,000.
If, however, you want a comfortable, swish long-distance express, then it would be hard to argue against the V8 diesel A8, with its thunderous 590lb ft of torque. The S8, in short, feels like a car that's ended up with the wrong badges; more an S-line than a full-on S model. And that's a shame, because the S8 is a car made up of many great parts. It's just that the sum of them doesn't quite add up.