I think most of the time attack S bodies run a 18 with a 295/30 tyre like this beast
http://www.speedhunters.com/2013/02/...r-suzukis-s15/
so for ultimate grip/track only I'd go with it must work for them,but as mentioned a 17 gives more sidewall as a compromise for road use
could even argue that a 18 diameter rim is more metal further outwards,so more rotating weight reducing steering and suspension movement (a tiny bit?)
but a small gain in ultimate grip from less sidewall/less tyre distortion 18 Vs 17
I doubt there is much in it tbh if you tested matching sized wheels with matching rubber just 18/17 back to back