I had an '04 Cobra, only mod was exhaust. I ran it at the track against 2 different gt500's and beat them both by .3 or more. The gt500 is a pig and you have to do a lot to make it really fast. Not to mention you get ford quality and service.
If you insist on a fast car to play around with and money is no issue, get an M3 and a 03-04 cobra to play around with.