I think Eddie just meant if you want the front and rears to match the size you need for the rear is 18mm. The M5 rear bar does not fit according to someone that tried it.
the 550GT's rear bar is 18mm so it should be perfect for those with the M5 front.
Current - 2013 N55 535xi Msport SAT
Retired - 2009 N54 335i Msport 6MT
Retired - 2000 M52TU 323i ASA Supercharged 5MT
More photos of my car here