My next move was so obvious that it would have been absurd even to speak of alternatives, much less give them any serious consideration. I had just picked up a Jaguar XF 4.2 SV8, and clearly I had to take it to the nearest network of properly challenging - and deserted - country roads and do some frolicking. So I did.But first I had to get there. That took about 40 minutes, and what became clear during that time was that although the SV8 is the high-performer of the XF range, it still feels like a luxury car when you're not trying to get anywhere in a hurry. In fact, it feels a lot more luxurious than the naturally-aspirated 4.2-litre V8 model, which I drove briefly earlier and which has considerably inferior ride quality thanks to a mismatch between the low-profile tyres and the soft suspension settings.The SV8 has even lower-profile tyres (sitting on 20" wheels, the largest available as standard on any XF) and the suspension is still quite soft, but the combination works better. If you had to choose between those two models and a cossetting ride was your main requirement, the SV8 would be the one to go for.However, that's not the point of this test, which I decided even before I opened the driver's door would be based primarily on how this car performs. The challenging-road stuff give me all the answers I needed about three of the SV8's most important features, and although I could discuss them in any order I'm going to follow convention and start with:The Engine. Nothing very new here. Jaguar has been offering high-performance models with a supercharged version of the 4.2-litre V8 for several years now, and that's what the fastest and most expensive XF gets too. No question, it's a fantastic unit, as docile as an anaesthetised kitten when you don't need to get anywhere in a hurry, but capable of producing 414bhp and firing the XF from 0-62mph in 5.4 seconds if that's what you want it to do.There are no holes anywhere. (I mean in its behaviour. Obviously there are physical holes, otherwise the eight pistons would have nowhere to go.) Whatever speed the engine is turning over at right now, you're only a prod of the right foot away from a visit to a place just short of the revlimiter, which you'll reach to the accompaniment of a wonderful V8 roar that would have seemed unimaginable at tickover.Along with that roar there's the whine of the supercharger, which is less tiresome than it was when Jaguar first brought the engine to production, but only because it has been muffled in the intervening years. Still, you can't allow yourself to pay too much attention to that because you should be concentrating on the fact that the next vanishing point in the road is rapidly approaching, though not as rapidly as might otherwise be possible because of the performance-sapping nature of:The Gearbox. It's a six-speed automatic, and it's probably the main reason why the XF is appreciably slower in a straight line than, for example, the Audi RS 4, which produces the same amount of power but has a more efficient manual transmission. (Yes, I know the Audi loses power through its four-wheel drive system, but I think I have a point here.)Still, there's plenty of performance left over, and the gearbox is a delight in itself. Once the innovative rotary gear control has emerged from the centre console - as part of a so-called "handshake" which also involves the air vents appearing from the dashboard - you can either let the transmission choose the gears for you, or you can pick them yourself using the paddles mounted behind the steering wheel.Either way, the gearchanges are wonderfully smooth, even at full throttle, but the manual option is definitely better in hard driving because it gives you more control. And speaking of control, let's turn to:The Suspension (and steering). The first thing to say here is that Jaguar has never, in the modern era, had the slightest problem in making the rear ends of its supercharged cars deal with power outputs on the high side of 400bhp, and in that respect the XF SV8 is right up there with its predecessors. This is very reassuring, certainly more so than the behaviour of the front end.The problem with the front is that the steering is superlight thanks to the level of steering assistance. Frankly, I think this has been overdone. Being able to change direction with the tiniest application of force through your fingertips is fine in a luxury car (which, as previously noted, the XF is, at least to some extent) but in a sports car (which the SV8 is too) it makes you feel too remote from what the front tyres are doing. You can see the view ahead rotating round you as you enter a corner, and you won't miss the lateral g forces, but there's no sensation of rubber digging into tarmac, and this is a mite disconcerting.On the other hand, the fact that that's exactly what the rubber is doing is beyond question. You turn into the bend, worry slightly that you can't feel what's going on up front, decide that everything must be okay, apply lots of power - perhaps all the power the engine can provide - and breathe a sigh of relief as the rear digs in and spits the car on to the next straight. Clearly both ends were doing their job properly, though you still need to take it on trust that this will happen again as you feel that apparent vagueness on turn-in to the next corner.One more thing. I mentioned the 20" wheels and those low-profile tyres several paragraphs back. In slow motoring the tyres pick up road surface imperfections, but not to an annoying extent. If you take the SV8 to a different performance level on anything other than beautifully-laid tarmac the situation becomes more alarming.Drive quickly over bumps and their effect will be transferred to the cabin so acutely that the steering wheel starts to vibrate in your hands, and in extreme cases the interior trim starts rattling. I've not found a situation in which the handling begins to suffer, but the feel and sound of all that shaking might make you question why you paid nearly £55,000 for this car.The SV8 is the "halo" model in the XF range, and there is no possible way it's going to be the top seller. The few people who are prepared to pay for it will find that it doesn't do everything right, but that it does a lot of things very, very well. Engine 4196 cc, 8 cylinders Power 416 bhp @6250 rpm Torque 413 ib/ft @3500 rpm Transmission 6 speed auto Fuel/CO2 22.4 mpg / 299 g/km Acceleration 0-62mph: 5.1sec Top speed 155 mph Price From £52779.00 approx Release date 01/03/2008