From my research there is two options for the rear. The M550dX's rear bar is 16mm and the F07 550GT Msport is 18mm. Stock is 14mm. Cars without sports package might even have a smaller bar but not sure. Going from 14mm to 18 is a huge difference. Joe@ptech is trying it out next week on his 550i so i'll wait and see what his finds are first. I may go with the M550dx's bar because i don't want the car too 1)decrease ride quality and 2) be too tail happy. My goal isn't to make this thing handle like a track car but just make it handle like how i've come accustomed to a Bmw should handle.