RC, thanks for the heads-up
The rims would be for normal driving, not tracking, and I a sure that missing another inch of rubber on a car that is already a stiff ride will make it anything but comfy - and might even put unnecessary strain on the roof and body.
I doubt it will crack under normal use, tho.

Im thinking of getting R compunds for my 18"s for the upcoming trackday...I think that might be the way to go...