I have the BMW Performance exhaust, and it puts a smile on my face every time I drive my car! I got a great deal at www.getbmwparts.com
, and because I picked it up in person, no sales tax or shipping. Installation took me a few hours, and I posted a DIY on here with lots of photos. It was the best mod I did on the car (well, second only to the KW coilovers) I did not go aftermarket because I wanted a sport exhaust while still under warranty. Power gain is small, a little more when combined with the performance intake, maybe 5bhp? Toque feels better too, and accelration is very nice.