It's often the case that a particularly high-performance version of a given model will be there to create a halo effect for the rest of the range rather than to achieve a significant number of sales. That, however, is not likely to be the case with the Jaguar XK in its 2010 model-year form. Jaguar itself reckons that half of the new XKs it sells will be in the form of the supercharged XKR (the rest being divided more or less equally between the standard and Portfolio versions of the naturally-aspirated car), so it seemed appropriate to introduce ourselves to the revised range by driving the R first.There have been several updates to the XK recently, and we'll look at most of them shortly. The most exciting, though, is the new AJ-V8 Gen IIIR five-litre engine, which produces a maximum of a little over 500bhp. That's a figure which other manufacturers such as Audi and BMW have already exceeded in their premium sports cars, but it's a new development for Jaguar, and one which has a rather startling effect.But not an immediate one. The XKR is fitted with 20" wheels and exceptionally low-profile Dunlop Sportmaxx tyres (35 section at the front, 30 at the rear), and the most obvious evidence of their existence is that they transfer even the tiniest road imperfections straight into the cabin without making any effort to soak them up first. But the suspension set-up is quite soft, and if you manage to ignore the jitteriness caused by the tyres you'll soon realise that the XKR feels like a luxury car should.This XKR also makes a nicer sound than the previous one. The old 4.2-litre R engine was a very impressive unit, but its supercharger made a noise that might have tempted an RSPCA inspector to open the bonnet and check whether a cat had crawled under there and got its tail stuck in the alternator belt.Eaton, which continues to supply superchargers to Jaguar, has not only made its new compressor more efficient (it takes 20bhp less to operate than the previous one) but also attended to the noise problem. Generally you can't hear the supercharger at all, and although a slight whistle does become apparent at full throttle it's as difficult to identify as trying to listen to someone playing the flute during a firework display.Not that you should be paying too much attention to the sound if you're driving that hard, of course. 500bhp is a hell of a lot for any road car, and if you use it all the XKR erupts in the general direction of Somewhere Else at such a rate that you're soon going to have to stand on the brakes, or at least back off the throttle. The difference between the 0-62mph times of this car and its predecessor is only 0.4 seconds (4.8 versus 5.2) but the generally impression is that a very rapid car has been tranformed into a shatteringly quick one.This is all a lot of fun, but I think something has been lost. The old 4.2-litre engine produced around 420bhp, and I used to marvel at the fact that Jaguar, which seemed to have difficulty making a 150bhp front-wheel drive X-Type feel remotely competent, could transfer nearly three times as much power through the back end of any car fitted with the 4.2 engine. I also wondered what you would need to throw at the rear axle to make it feel as if it had a fight on its hands.The answer turns out to be something less than 500bhp. The new XKR doesn't exactly struggle with the extra power, but it can't handle it elegantly. All the supercharged 4.2 models clearly had to work to cope with 420bhp, but that work was so skilfully done that the driver was hardly aware of it.At this new stage we have reached in the arms race among manufacturers of premium high-performance cars, you certainly are aware of it; if you're driving hard there is enough tail-end squirming, even before the (entirely necessary) traction control kicks in, to make caution advisable.For some people the extra straightline performance will overwhelm any objections of this kind, but I think a certain grace has been lost, and surely a Jaguar without grace is no kind of Jaguar at all? Perhaps the time has now come for the XKR to be given four-wheel drive so that it can regain its former poise.But of course it depends on how you think the XKR should be treated. To me, it works best not as an out-and-out performance machine, but as a car which can be driven quickly but smoothly without using anything like the engine's full potential (and it does seem to be much better-behaved in more gentle motoring than the previous XKR was) in the knowledge that there is phenomenal acceleration available to you if you want or need it.As hinted several paragraphs ago, it's not all about the engine. Like other XKs, the R has been mildly restyled for 2010, with new front and rear bumpers, new door mirrors and revised exhaust tailpipes and side vents.Inside, there are heated and cooled seats, new veneers and - borrowed from the XF and appearing for the first time in the XK - a rotary gear selector which, as part of the "Jaguar handshake", rises from the centre console as the engine starts.It is not, as you can imagine, an inexpensive car, but Jaguar likes to point out that at £72,400 it is cheaper than obvious rivals like the BMW M6 Coupé, the Audi R8 4.2 R tronic, the Porsche Turbo Tiptronic and the Mercedes-Benz SL 63 AMG (all of which, apart from the Mercedes, are less powerful). Its official combined fuel economy figure of 23.0mpg is superior to those of the cars mentioned, and the CO2 rating is correspondingly "low" at 292g/km.If you are thinking of buying any of these cars, your choice will most likely be based on things like brand loyalty and self-image. You might pick the XKR simply because it is a Jaguar, or you might avoid it for exactly the same reason. If you are of the former mindset, the good news is that this is at least as glorious a Jaguar as any that has preceded it, and if you disagree with my personal opinion that it now has more power than is good for it, I wouldn't argue strongly against you. Engine 5000 cc, 8 cylinders Power 510 bhp @6500 rpm Torque 461 ib/ft @2500 rpm Transmission 6 speed semi-auto Fuel/CO2 22.5 mpg / 292 g/km Acceleration 0-62mph: 4.6sec Top speed 155 mph Price From £76380.00 approx Release date 01/04/2009