When my wife's 2011 128i Convertible was delivered, the BMW PE I ordered for it came in at the same time. The dealer wanted $220 to install it, even with my BMWCCA discount. I took it home and, with the help of a friend, removed the stock muffler and installed the PE within about an hour. I measured carefully and used blue tape to mark where to cut, and even removed the rear valance for more room to work. I used the following DIY link:
http://www.1addicts.com/forums/showthread.php?t=450937