According to what I've read, the offset requirements on 08 is much more lax than it was with the older cars. Much less likely to rub. Don't really have any links to back that up but I think I read that about the Cobb or Perrin project car, can't remember really.
It lets you compare wheels/tire sizes to see how it will affect your car. To maintain the stock height (and therefore accurate speedo, geometry etc.) the correct tire size is 225/40/18. Note that the sidewall on this tire is a half inch shorter than stock, so the ride is going to be rougher. I personally am going to stick with 17's. The sidewall height looks aggressive enough for me, and with a drop the stock setup fills the wheel wells nicely. I'm just waiting for more springs and struts to come out before I jump in.