I have a GTI now, but I could always see myself getting an R in a couple years. If I were to get 17" wheels for the GTI, would those fit on the R? Or would I have to get 18"?
You should never mix dimensions on an R because of the 4 wheel drive. You will destroy it. So if you want your seventeens from your gti to your R be sure to get new tires so the diameter is the same or you'll have your car broken down...
The Haldex can accomodate up to about 3% variation between front and rear diameters, so if one pair of tires is worn a bit more than the other it isn't going to grenade the Haldex.
Also, several MKV .:R owners have been running staggered setups for some time without issue; so minor diameter variations aren't going to kill you