used to have an 06 sti running stage 2.5 with a tune. Intake/header/exhaust all 3" piping. I've walked stock 335i's and an 135i from a roll and dig. Any mods on the 335 and it would be a close race till 130 km/hr, then the v6 would pull away.
In terms of reliability, although it looked ricer from the outside, the STi had heart and extremely well built. I could see modded cars running easily over 200k kms without any issues. Very well built cars. The tranny is good for 600whp.
The "tuning issues" were sorted out via dealership reflash. Most people flashed to an aggressive aftermarket tune anyways, and the car runs rock solid.
Out of all the cars i've had in the past, the sti had the most personable driving experience.