There used to be a place in holland that delt with german performace but I think it may be out of business, autoworks or something i think it was called. I don't know much about their work though. There is a guy around GR that has a heavily modded M5, I met him once, he'd be the guy to talk to. I'll see if i can get a name or e-mail for ya.
99 SI | 02 Maxima 6-speeds | 02 CBR F4i | 09 CBR 1000RR Repsol | 09 370Z Nismo