Equ,
When I had the Z4, I put four nrf snows on separate wheels (17's) - the ride was quite different - the combo of a smaller wheel, higher profile and non runflat construction made for a different experience.
Why not try this with winter coming? If it works out for you, then get rid of the 18 inch wheels and fit four nrf's on a set of attractive lighter weight 17's for summer use.
FWIW, the 18 nrft I run with on the ZHP are fine in all kinds of driving, surprisingly so, I even traversed the Willis Ave bridge without trouble a couple of weeks ago
I'll be putting the 17's on the ZHP in about three weeks (from the Z4), it rides even better all winter with this combo.
Ed
Ed