Here are a few screen grabs of the M Sport package from the 2013 ordering guide. I think it may be the difference in tires that explains the top speed limiter difference. xDrive: all-season tires, RWD: performance tires. Maybe it's a tire speed rating thing. I don't plan on taking the car much above 130 anyhow, but it still sucks!
650i xDrive
650i RWD