Maybe an obvious question but are you thinking of fitting non run-flats as they should be much better than run-flats.
My car had 19" CSL Replicas with 265/30 rear tyres when I got it but I have swapped to OE 224s and went for 255/45 x 18 rears instead of the OE 255/40 size and it has made a big difference to ride quality!
I've had no issues, and my speedo is actually more accurate.