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, 2013 550i M Sport
Gone but not forgotten:
2006 2.0T A4 Avant, 2007 Z4 Roadster 3.0si, 2006 M Roadster