No 17" wheels are offered (at least from the factory for the sky), they would be aftermarket. 18's would look best IMO because they fill in the wheel well and the car was made for them (hence why they come as standard equipment), 17" may throw off the tuning of the suspension (just like when you upgrade wheels on a car with 14" and go to 17 or 18") but to his his own
If you're autocrossing, 17's will put you OUT of any stock class.:nono: However, in the street tuner and Street Prepared there would be a tremendous advantage.:thumbs: Tire costs go down, wheel size choices go up and 17's are cheaper, rotational weight goes down, and the car will be lower.:yesnod: If I ever go Street Prepared from A/Stock I absolutely will be on 17's.:driving::drool:
There seems to be a bit of confusion on wheel size, I see things a bit differently. Basically you want to keep diameter (tire) the same whether you have a 17" or 18". The only way to do that would be to adjust the aspect ratio. That would mean a higher aspect ratio on a a 17" wheel. With all that said you will lose performance on the 17" tire with a higher aspect ratio.
If the car came with 17's and you wanted to change it, that would be different, but, in our case 17's would be a step back. The exception would be drag slicks, in that case a 16" would be advisable.
If you don't mind getting pushed out of a stock class in autocross, 2.85 x 30 sounds kind of interesting on the rear. We are going to have find out how large the fronts can go without causing the car to track abnormally.