Personally I'm surprised these haven't taken off more than they have, best option IMO if you're not planning on upgrading to bigger brakes, and even then some smaller kits probably fit under the 17's. We're talking a 40-or 35 series instead of a 35 or 30 series for aspect ratio, it's not as if the 17's are balloon tires, and a little extra sidewall in the rear doesn't hurt for forward bite either. A 275 R888, for instance, is $51 *per tire* cheaper in 275/40-17 vs. 275/35-18. And there are a bunch more options in wide 17's in the semi-slicks like DOT Hoosiers that still run in the 25-26" diameter range that this car needs, and they're once again $25+ cheaper per tire than the 18" sizes
There is plenty of room for a 315/35-17 in the rear, though I'm not sure if the offset on the ET27 17x10.5's will get the job done. Only one way to find out...