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.
2014 LCI 550i M Sport | 3D Design Front Spoiler | BMW Performance Black Kidney Grills | BMW Performance Trunk Spoiler | BMW Performance Gloss Black Diffuser | Painted Reflectors | Wincos 30% Tint | Supersprint Quad Exhaust | AC Schnitzer Springs | MORR VS8.2 Brushed Finished Wheels 20x10R and 20x9F | Michelin PSS Tires 245/35/20F 285/30/20R | M550d 16mm RSB |||More Photos of my car here|||