Have you thought about a Noble? Your looking for a track car primarily and that is an amazing car. You can get them over here as a kit, ie motor and car separate so they don't have to deal with us regs. Plus you will probably never run into another one here. http://en.wikipedia.org/wiki/Noble_(car)
what about the ariel atom? you can actually buy them now in the US from I think a place in Oregon, or you can get them in kit form. Top car ~300hp in a 1200lb car. The top gear video on the car is hilarious. http://www.arielatom.com/ David
i think a radical SR8 would be ridiculous to own, two suzuki hayabusa shortblocks mated together to form a 2.6L V8 with a 10,500RPM redline. Most awesome track car. http://www.radicalextremesportscars.com/news_folder/dvd2005/sr8.php oh yea, also, generates more downforce than it's weight...so if you go fast enough, could drive on a wall/ceiling.
I've considered both the radical and the Ariel Atom. I'll probably buy one or the other at some point. The Radical appeals to my boy racer side, but the Atom is a cool street-able car.