repeating above...
The best place
would be a BMW dedicated messageboard... I would suggest starting at
www.bimmerforums.org first, and then branch out.
I have an E30, which is an M20 engine, and I only know about chips for M20's and M10's.
Your M50/M52 is unfamiliar to me.
And my chip (MarkD, if you speak the E30 language) did give me a dyno proven 18 horsepower, and 20 lb/ft more than when it did not have the chip... but then again... I have a 325e, and they are intentionally restricted from the factory for efficiency over power.... not anymore.
325i's (M20 engine still) gain on average about 7-11 horsepower, depending on what you've already done to the car. A good combination would be: No catalytic converter, aftermarket exhaust (Ansa

), and Cold-Air intake, chip, shorter differential.