Most of the guys that have got the 35i either go for Dinan or JB4.
Heres a thread where the JB4 has been installed and its review:
http://f15.bimmerpost.com/forums/sho....php?t=1129993
r33_RGSport would help you better.
Personally I've driven the stock 35i and it needs more work. I think even a race chip might be enough instead of going all the way to full tuning, and it's cheaper.
Best of luck.