You can drop from 19" to 18" and get a lot of ride comfort back. I'm amazed every winter when I mount up my snows how much nicer the ride is with the 18" wheels and tires.

However, tire choice is a big factor too. On my Sport with the 19" wheels, swapping from the stock Michelins to my new Bridgestone Dueler HP tires, the difference in ride quality is significant. While the Diamaris tires were always harsh, noisy and lousy in the wet, the Bridgestones are comfortable, quiet, and nice in the rain, while being every bit as good at dry handling.

__________________
'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R