If possible, you want to avoid using runflat winter tires. They are way too stiff for optimal performance. I purchased Blizzak LM60's and mounted them on my stock 18's. I went slightly narrower than OEM... 225/50/18. They're a relatively smooth ride considering they're winter tires....... You definitely don't want to pass 75MPH on these things though. The car remains controllable, but the stability factor is just not quite there. Throw non RFT's on and carry a few cans of "fix a flat" in the trunk.
Purchased them from tirerack and had them shipped to my local STS. They did a wonderful job... charged 25 per tire to mount and high speed balance.
Here is a crappy photo of them
