In retrospect it seems very strange, but when the chance came to drive the Porsche Panamera GTS round Knockhill race circuit I needed some persuasion to take the wheel.It's not that I don't like driving on race circuits. Quite the reverse. Nor do I have an objection to Panameras. But I'd already been out in a Boxster, and I was waiting for a 911 to come back into the pit lane. The Panamera is a luxury coupé with roughly the same road footprint as a Ford Transit, and I would be taking this one out on the public roads soon enough - right now it just didn't seem like a car I should be devoting much attention to.Ah. Yes. But. The thing about the GTS is that it's the sports car in a range that you wouldn't normally consider sporty. It's not the most powerful Panamera overall, but with 430bhp available from its 4.8-litre V8 engine it is the most powerful of the non-turbocharged cars.More importantly, it also has revised suspension (using air rather than coiled metal as the springing medium), a ride height 10mm lower than that of other Panameras, brake discs otherwise fitted only to the Turbo and the Sport Chrono Package which looks after settings for the engine, seven-speed semi-automatic PDK transmission and damping in three modes called Normal, Sport and Sport Plus.As if that weren't enough, the test car was fitted with two very important and dizzyingly expensive options. One was the combined Porsche Dynamic Chassis Control and Porsche Torque Vectoring Plus, which respectively employ active anti-roll bars and divide power between the rear wheels in the most appropriate manner for the conditions. That costs £3197, a figure dwarfed by the £5924 you're asked for ceramic composite brakes.I realised the value of the brake package almost immediately after being persuaded by the Porsche people that I might as well get busy with the GTS while I was waiting for the 911 to come back. The colleague who had been driving it before I did had clearly been doing so with some purpose, as I suspected from the smell of the car and was convinced of when I discovered how far I had to press the brake pedal before anything happened. If he'd been pushing that hard without the ceramic composite option there might not have been any brakes left at all.Moral: if you're going to do trackdays in a GTS, spend the extra £5924.In all other ways the car was surprisingly, almost shockingly good, as became clear within half a dozen laps. Knockhill, as you'll know if you've seen TV coverage of races held there, is all about the corners. Unlike many circuits further south, it offers few opportunities for a powerful car to exploit its straightline potential to the full, and before driving this one I'd have thought that Silverstone, for example, would have provided a better environment.The realisation that this ain't necessarily so came in three stages. First, "hey, this thing isn't floundering all over the place." Second, "it's actually quite nimble for its size." Third, "good grief, you can chuck it round like a hot hatch. How is this possible?"And even that wasn't the best bit. The best bit is the fact that the GTS, much like the latest-generation 911 and Boxster, is also amazingly smooth and comfortable on the public road. I'm absolutely not exaggerating when I say that this very same car could be used to transport an elegant couple, sitting in no small luxury in the very spacious rear passenger area, to the opera house and back without complaint, and perhaps with compliments.This exceptionally satisfying car is not even close to being the most expensive Panamera. At £91,239 it's more than £32,000 cheaper than the 550bhp Turbo S, a car which is inevitably more dominated by its engine than the GTS is. With all the options fitted to the test car (those mentioned above, plus 20" wheels, red metallic paint at a frankly scandalous £2517, a £1708 black interior trim package, heated seats, privacy glass and a universal audio interface) it costs £106,398, and despite Porsche's habit of charging as much as it can possibly get away with even this sum is among the more justifiable in the Panamera range. That's how good the GTS is. Engine 4806cc, 8 cylinders Power 430bhp Transmission 7-speed semi-automatic Fuel/CO2 25.9mpg / 256g/km Acceleration 0-62mph: 4.5 seconds Top speed 179mph Price £91,239 Details correct at publication date