Here are a few inspirational shots of the Hamann overlay diffuser on a e92 that is small and simple, yet looks good wrapping just around the area where the exhaust outlets are. No need for a large footprint spanning the entire back IMHO as long as the envelope follows the lines of the lower bumper.
http://s1002.photobucket.com/albums/...pe-hamann3.jpg
http://s1002.photobucket.com/albums/...pe-hamann5.jpg