I run the UK 535i M-sport wagon on the standard fit 18" (Styling 350) wheel set. I've driven a 535i sedan on 19" and find the 18" more refined.
My 535i is optioned with Adaptive Drive, (DDC & ARS), I've driven a 530d sedan with 19" and Adaptive Drive and still feel the 18" give the better ride and refinement. I sense a bit more agility on the smaller wheels, but that is a subjective issue anyway, as RFTs are not the best bed fellows with suspension purity. From my reading and discussions, AD was designed around the 18" wheels anyway, so likely to give optimum performance.
I will be going over to my 17" winter wheel set this week.
"Form over fashion" for me.