I doubt if going up one profile each end would cause a problem.
The standard tyre sizes on the Z4Ms was 225/45x18 on the front and 255/40x18 on the rear, although for much the same reason as you I had 255/45x18 fitted on the rear of mine and haven't had any issues!
224s Fitted 230421.jpg
It will alter your speedo, but given how all my BMWs have over-read it will probably just make it more accurate. :lol:
But if your car is still on run-flats probably the best thing you can do for ride quality is to get rid of them whatever sizes you decide to get!