IIRC blueflame is an ebay company no? Not sure if they are the same as powerflow etc, but ive heard bad reviews for BMW's having a non tested system fitted. The likes of Supersprint and Miltek etc take a lot of time on R+D, which then you will get performance increases not decreases.
Might all be cr*p TBH, as ive seen some on E46's and they look pretty good! But personally i would get a Miltek or similar quality exhaust.
What you after backbox only, or cat back, downpipe back?