Not sure what you mean by tallest sidewall. I've seen 24" rims, but then the sidewalls are what I would call very short... 25, as in 275/25ZR24. The Pirelli PZero Nero 275/25ZR24 are 29.4" in diameter.
Yeah I beleive the 18" option was with the G package, maybe not, but I have the 18's on mine.I think you would be OK with that tire. It's the same wheel well that houses my 245/45ZR20's. Incidentally, now that my rear tires are near the wear bars, I will be upsizing to a 255/45ZR20 which was an option for the rear anyway. I want to stay with the Goodyear RSA because I love the ride. Wasn't an 18 inch rim an option for the SE? That could be another way to fill up the wheel wells.
Jack