Someday I might give 18" a whirl, but I already have far more handling for the roads than I would need with 255/40R17. It's pretty jarring but that has as much to do with my Ohlins as anything else. Actually, come to think of it, tire makes a difference. I used a set of Advans Neovas, the ride was a bit smoother but they wore out extremely fast. I have Kumho Ecsta SPTs now, and although they spin more going on launches (meaning they grip less and hence make the car a touch slower on take-off), the car feels more stable in hard cornering (even if max lateral Gs might be a touch less, I have a lot more feel and confidence). But best of all, they LAST. I bought my tires three years ago, rotated them last year, and I've only used about 2/3rds of the tread.
Oh, and they are worlds better in bad weather - rain/sleet/snow. They're no Blizzaks mind you, but certainly not greasy like the Yokohamas were in the rain.