I THINK I’LL DO the disappointing bit first. It’s best to get it out the way.Here goes, then – the S8 may have a V10 engine sourced from a Lamborghini but it’s not Audi’s answer to either the BMW M5 or Merc S55 AMG. When all is said and done, it’s just not that big a hitter.There, I’ve said it. Now we can move on to what does matter. Those who buy an S8 when it goes on sale in June for around £70,000 will find they have bought one of the most satisfying big saloons ever conceived.It is tempting to be seduced by headlines like ‘Lambo engine’ and ‘444bhp’ but this is not what the S8 is about. The S8 is about distances disappearing for no apparent reason; it is about how outrageous performance can be sampled yet controlled, delivered yet contained. It is a Bengal tiger in a very strong cage – colossally powerful yet no more capable of biting your arm off than next door’s marmalade mog. Above all an S8 is about owning, even more than it is about driving.Disappointed? I was too, but only at first.You see, I was seduced by the Lamborghini thing, forgetting that it is the Gallardo that has the Audi engine, and not the other way around. And yes, I was hoping it might explode down the autobahn like the S55 AMG I drove not two weeks ago.I had no right think such things because the Merc is £20,000 more expensive, but that’s the risk Audi runs when it wheels out an ‘S’ badge and starts sprinkling the Lamborghini word over its communications.But the figures soon reveal a less startling reality. Despite the V10 being bored out to 5.2 litres, its power has been pegged back from the Gallardo’s 520bhp to the aforementioned 444bhp. Audi’s engineers say this was required to reshape the torque curve into something more practical to owners who will use their S8s as limos most of the time.But one did admit darkly that big-number versions of that engine all had Lamborghini chaps on their toes. It has 398lb ft at 3500rpm which sounds impressive in isolation (and beats the Lambo motor by 22lb ft) but the bar is set by supercharged AMG E and S-class Mercs that register over 500lb ft.Yet what really relegates S8 performance from the life-changing to the merely life-affirming is, of all things, its weight. Would you believe the S8, with its lightweight aluminium spaceframe construction and lightweight aluminium body, actually weighs 30kg more than the big old S55AMG with its steel monocoque and panels? It’s true. Or that while the equally aluminium Jaguar XJR may have only 400bhp, it is also an astonishing 281kg lighter? You can pile a family of four into the XJR and it will weigh less than an empty S8.So put the S8’s ability to hit 62mph from rest in 5.1sec, compared to the Jag’s 5.3sec, down to the traction of its four-wheel drive system (reconfigured for the S8 to send 60 per cent of the engine’s torque to the rear wheels instead of Audi’s usual 50:50 arrangement) and not its inferior power-to-weight ratio.Indeed, we must remind ourselves that not only is this far from the fastest car in the class, it’s not even the fastest A8 in the class — the W12 A8 costs only £3500 more, and is claimed to have identical performance, yet its 6.0-litre engine has a useful torque advantage – meaning the best the S8 could hope for is a dead heat.But we should not allow ourselves to be diverted by such issues. What car, let alone which Audi, could ever be judged by such bald statistics alone? And as my time in the S8 ticked away so, despite filthy weather, overcrowded roads and a tight schedule, did my admiration for its subtler charms start to blossom.I liked the fact that the S8’s ride is at least no worse than that of a standard A8. It might be somewhat better. At a steady 100mph, the interior is limousine quiet and while I care not at all for the italicised S8 dials, the rest of the cabin, from the carbon dash and aluminium fillets to the supportive sports seats, create exactly the right tone of sporting luxury.And then there’s that engine. I’d rather listen to a good V10 over most V12s. I find their offbeat thrum quite fascinating and this one, with its immaculate low-speed manners and cultured high-rev yowl, is one of the best.It doesn’t bellow at you as it does in the Gallardo, but Audi has judged well the character of the S8 and tuned the engine to suit it to perfection. Keeping your foot to the floor and banging through full throttle changes via the steering wheel paddles is an addictive pursuit, so fine is the noise and so strong the thrust.Oh yes. Do not mistake me, for this car is quick: not for the first time Audi seems conveniently to have forgotten to install speed limiters on its pre-production cars and I found one stretch of unrestricted autobahn sufficiently long and deserted to know it still pulls hard at 170mph, a speed at which its only other true rival, the Maserati Quattroporte, gives up. Thank class-leading aerodynamics for that.Find some curves and S8 does feel sportier than other A8s, but it’s still a long way from the indulgences on offer from the rear-drive mob, despite the lowered ride height and stiffer springs. As with all two-tonne cars, it’s better in faster corners, and if you can find space to crank it over at a decent speed, you’ll find excellent composure, impressive grip, seemingly limitless traction and, of course, understeer.The steering, with slightly raised gearing, is typically Audi: precise, direct and predictably lacking in feel. The test car was also fitted with £5500 of ceramic brakes which frankly felt exactly the same as the steel items on other A8s, but should last the lifetime of the car and drop the corner weights by a very useful 7kg.It is easiest to define the S8 by what it is not. As I said at the start, it is not a rival for either M5 or S55. It’s not the world’s stealthiest supercar nor does it mark any great change in thinking from Audi. Like all fast Audis, even the RS4, it’s most comfortable in the role of the efficient and effective ground coverer, and while I have damned others for the tedious practicality and worthiness of this approach, I feel disinclined to do the same here.What the S8 does do is expand the already mighty talents of the already mightily appealing A8. It takes all that civility, style and taste and gives it an edge. While others might be faster and some might even be more fun, the S8 is without doubt the only big German saloon with a legitimate claim to being cool.In an age where saloons are now perceived to be the least cool things on the road, that is some claim to fame. To those shopping for a car like this, that is likely to prove as compelling an argument in its favour as any amount of extra urge boasted by others they might consider. And if that tipped them over the edge and into its arms I, for one, would not blame them for an instant.