The quest to drop weight from the M3 continues. A Borla Cat-back Muffler has been installed. The Turner web site states that it will result in a 5 hp increase, but I am just as interested in it being 26 lbs lighter than the stock system. It’s built from 
T-304 stainless steel (inside and outsize) and comes with a 1,000,000 mile warranty. It is a bit louder than stock, but not so much that it will wake the neighbors.
Installation was quite easy. I consider myself lucky that the bolts for the stock exhaust, although rusty, were removed without incident.
Here are the "before and after” videos (taken with my iPhone 3GS).
Lang Motorsport 