i would recommend a 17 or smaller.. particuarly if youre planning on doing any autocross or racing. Even if you arent, low profile tires require a lot more attention while driving! I blew out a tire in my town's center around a manhole 2 weeks after getting my rims on.. the tire size was 215/40/17.. I could have got a 215/45 also. If you do plan on getting into competitive driving, get the rears skinnier to reduce understeer. I'm not sure what you should get if you go 18, but I strongly advise you to get a 17 or smaller.. Even a 17 is pushing it on these cars. If I could do it again I'd go 16. If you want to get more size recommendations, go to tirerack.com and select tires for your vehicle, and choose an "optional size" of 16, 17, 18 and it will tell you which tire sizes are most common and recommended...
think about this... how much low pro do you need???

215/40/17---


good luck, I hope you value my advice.. hindsight is 20/20.
drive carefully..
mike.