SCCA rule basically says you MUST keep stock rim size (you can swap out for lighter rims of course) to stay in stock class, but R-compounds (Victoracers, Hoosiers) ARE OK, and is probably THE biggest enhancement you can do in stock class (at least 3-4 second difference for a good driver)

change of rim size should still keep you in STX. You have to do A LOT more than just rim size to be bumped to SP classes... (like stripping out interior trims, removing seats, etc...)