I was once interested in a 6 speed but from what I know now it is totally useless if you are going to power up your motor.
For anything with a decnet amount of low-rpm grunt, I like to think of the F40 as a five-speed + low, not a six speed.
The only real appeal it has for me, is its "out of the box" abuse resistance. Well that, and the apparent lack of frame mutilation required to fit it in the engine bay (unlike the NSX trannys).