Yeah but the difference is, if you tune an ultima engine and it goes pop how much will it set you back? I know you can build an awesome Honda engine for £2000 (not including engine)
so you can get an elise or exige down to 700kg probably more 760 with a honda engine, 500bhp or more, thats some serious power, theres a program where you can work out power to weight ratio somewhere. You won't need much more than that power for track use anyway. Just think the Veyron has a p/w ratio of 500bhp/t the exige is well over that, its cheaper, not a kit car, but you'd still be more or less rebuilding it from the ground up.
A well built one could crack 200mph, plus the possibilities of engines you can put in it are endless.