I believe the GT-500 is electronically limited to 155 mph or so. I can't remember.
If you're looking for a classic American performance car for sustained high speed cruising, then you probably want a Corvette, not a Mustang. You can buy those in Europe...top speed is 186 mph (at least) for the base C6.
It also has the brakes, suspension, electronics (active handling, variable power steering, etc.), large gas tank, and aerodynamics to be driven right up to it's top speed.
If you do get a Mustang GT, then I'd at least upgrade the brakes. The car is fine for cruising up to and around 100 mph, but if we're talking sustained (not short bursts) speeds of 120 - 140 mph, then I'd want bigger brakes at the least.
The BMW M3 has an electronically governed top speed of 155 mph. Imagine blowing by one in a Corvette doing 160 or 170 mph and knowing there is nothing he can do about?!
Anyway, check out youtube!