Thanks for your reply man. Yeah I get what you mean but truthfully, I just wanted the 255/275 size for the looks; to fill the arches more. I mean I use the car, don't get me wrong, it's not like I have an M3 just for show but I wouldn't mind sacrificing a slight bit of ride comfort/grip for the better profile. If I tracked the car then it would be a different story - too bad BMW SA won't allow that.
If experiences are that stock sizing would be better then I'll reconsider that again. It's just for the price I thought I might as well upsize (the 265 sizing actually costs more than the 275/285). I may also want to run spacers (say 15/12) in the near future but from the other threads I believe the sizes I mentioned above will pose no problems?