R makes a lot of difference. The crazy thing about AutoX is that once you jump up in class you should be prepared to make all the allowable mods. If you don't, you better believe the next guy is, then you can't hang with them. If you're in a class that allows R compounds then go for it. You can get a cheap set of rims at tirerack and wrap them in Kumhos. If you get on autocross.us there are plenty of people on there willing to help.
Have fun out there. I know I do.