thats another reason to get the best match for your car.. a 215/40/17 is slightly (.1") smaller overall diameter than stock.. I would avoid getting a bigger one. It throughs off your speedometer, puts more stress on the tranny, slows down accleration(increases top speed.. but doesnt matter).. theres just no reason to get a bigger combo. If you are SURE you dont want to get into driving like I am.. then go for 17s, but drive careful. I would prefer a 16 if I could do it again. 17 has that low pro appeal.. but at the expense of performance (smaller air chamber), weight(mine are very light at 17lbs a piece, but smaller wheels would be even lighter), and blowout potential. Make sure to find a tire with a stiff sidewall also if you intend to get into autocross etc. It will handle better, but it will be a little stiffer.
Think twice about buying big wheels! And my advice is to NOT buy wheels into you have tires and vice versa. Tirerack is a great resource.