Just ditching the runflats will smooth out the ride a LOT without downsizing to 17's. I did this on my E90 330i back in the day, and I could feel the difference before I even got out of the tire store's parking lot. Smoother ride, better handling, less unsprung weight. A complete win-win, other than the loss of runflat ability - but that's what a cellphone and AAA card are for anyways.
'08 Z4 M Coupé LM08602
'19 Golf R