I'll be using 30-series, 18" tires for my autocross set up next year. I'll be going to a 10" to 10.5" rear wheel to allow as much tire as possible back there, to gain acceleration, since AX is mostly run in 2d gear and I now spin my 19" wheel/tire set at will anywhere on the course. The low profile will give both a wider contact patch and stiffer sidewall. The front tires will also be 30-series and about 20mm narrower than the backs, to maintain balance.
The added advantage of 18", 30-series tires will be that the final drive ratio will be slightly higher numerically, improving mid-gear acceleration.
For street, I'll be staying with the 19" factory option, forged wheels in summer and 18" factory wheels, with snow tires for winter. I think these are the best compromise of ride and handling, where my AX setting is aimed at acceleration and handling with no concern for ride.