Who says the F50 is necessarily faster than an F40?
In fact, in straight line acceleration, the F40 has been known to outrun the F50. Of course, some say the F50 can turn faster lap times, but owners that I have spoken to, who have owned both, say this is very very debatable. It's only because the F50 is more docile.
The F40 is THE closest thing you can buy to a street-legal racecar-- that and the McLaren. It is also designed to be tuned to 700+ hp by simply swapping out the turbos...
Let's face it, the 911 just doesn't play in this league. Only the GT1 and the Carrera GT have a prayer at playing with these boys.
Also, the 0-200km/h time of the GT2 is 12.9s, almost a full second slower than the F40!