just tried this mod out for kicks.... IMO its actually quieter at startup, and only a little louder around low rpms.. The only time I was impressed is when putting around 1800rpms in 4th gear, flooring it gives a nasty bark... Up top it sounds the same though.
No noticeable power increase/decrease.
I was hoping for more i guess. I think the E90/92 335s have a much nicer sounding factory exhaust, plus the golf tee mod makes a more noticeable difference for them.