Originally Posted by mhabs
BMW actually puts real engineering work/R&D/wind-tunnel tests these add-ons prior to marketing. For this and the front splitters, BMW claims that there is additional downforce generated (per original press releases). While probably not noticeable especially at legal speeds, a even a few lbs of downforce or lift reduction is a win, plus you can't beat the OEM quality/looks that you want to retain with a $65+k car.
I love mine, looks great. And I don't presume that its giving me any real added downforce in corners like a wing would, but I DO feel the lift reduction on the long backstraight at Road Atlanta where you can get noticeably light over the last rise at 140. Now it feels rock solid. I think it definitely has some functionality along with the aesthetics.