It goes around 215mph on default. But you can tune the transmission to increase it's max speed. The FGT is a very fast car. However, it's a bit tricky to handle it.
1) You have no choice but to buy it, really. There is one series of races in which you can only use the Formula GT. So, you either buy it or you can't do some of the races.
2) It is allowed in many of the later races, in which it's the second-most dominating car behind only the X2010. So, if you want something that will massacre the competition, the FGT is your best bet besides the X2010.
3) There's a lot, lot more to vehicle performance than just the top speed. If you go by top speed, nothing would ever beat the Bugatti Veyron, but in reality there are several production cars that lap faster than the Veyron and heaps of race cars that blow the Veyron clean out of the water. The Formula GT probably won't max out quite as fast as the Veyron, and definitely not as fast as the X2010 (nothing matches the X2010), but the FGT will post faster lap times than anything else (to the best of my knowledge; can't think of any exceptions).
