There isn't any problem switching between the stock 19s to 18s, as long as the tire is appropriately sized. Probably the worst thing that can happen is maybe your speedometer would read slightly different. My current set-up is 18s with PS4Ss for daily and the stock 19s with Federal RS-RRs for track duty.Would it be a problem going from the stock 19s on the 18RS to a smaller set of 18s? Looking to run an aggressive summer/track combo, but then switch out for 8 or so months a year to a an aggressive all season tire.
Considering the tires will be the same, not giving anything up at all. Switching to 18s will be more economical, should open you up to more tire options and ride better.Anyway, are you giving anything up in terms of performance on the track going from the 19inch Cup 2s on the stock wheels to the 18inch Advanti Hybris on the same cup sport 2s? Is it a rougher ride?