I think that it does depend on the suspension rather than on the size of the wheels...
Having the same springs, changing wheels from 17" to 18"... shouldn't change that much the behavior of the car...
That said, when taking a sport suspension, the car does benefit from different springs (stiffer ones!) that do lower the car by 15mm... that change is noticable, even more noticable with 18" wheels...
One thing is sure... whatever your choice is, it will still benefit from a BMW efficiency i.e. a 1st class one
