I would say the weight of the 18's would be the biggest factor. I really dont think 17's or 18's are ideal for 1320 racing. If you have a heavy replica on street tires and a lightweight 18 with a DR I might say the the 18 will run better.
Sidewall will be an issue but your not going to get much sidewall diff. on a 17 to 18in rim to talk smack about.
Id run the lightest weight wheel possible with the a compromise on thick but not too thick of a sidewall. I know this wasnt a mustang issue but my Swaped CRX was a hell of a lot slower on 16" 16lbs rims vs 14lbs 15" rims....but then a honda has no toruqe so rotating mass plays a big role in its 1320 times lol