Beware that the rotor size on 550i are larger therefore only certain 17" rims would fit. 18" would give you more flexibility on rim styles and fitment.
2015 Cayman S
Gone but not forgotten:
2006 2.0T A4 Avant, 2007 Z4 Roadster 3.0si, 2006 M Roadster, 2013 550i M Sport