Re: GT on 18" ... worth considering?
Those are just my two Eurocents, but.....
On two cars I owned previoulsy I changed the wheel size, in one case going from 14 to 17 inch, in another from 16 to 19 inches.
First car was an E30 325 BMW, the second was a W124 500E Mercedes, in both cases, and in my very humble opinon, the loss in ride confort was negligible, and far compensated by the better handling. The ride was a bit firmer, but it's definitely not comparable to say a change of springs.
With this being said, my GF's Mini Cooper S is shod with 17's, and the ride is quite noticeably stiffer than an equivalent model on 16's, but the 17's are runflat tires, which I think account for the extra stiffness.