Just make sure the 17s clear the brakes if you have the Performance Package. then 245/40-17 Bridgestone RE-71.
For 17s, try to get you wheel weight around 20lbs or less.
For 18s, again RE-71s, 235/40-18. Sadly no one makes a decent autocross tire in a 245/35-18. Michelin SuperSports come in that size and are a wonderful tire, but the sidewall is too soft for autocross. Figure they are about 1-2 seconds slower per run than the RE-71. Asfor wheels, <21Lbs is a good goal. If you have the money, OZ makes wheels for the Mk VII in the 16-17 Lb range, but bring the big checkbook.