Just came across something in the rule book that I didn’t know about that reminded me of this thread.
“Cars listed as eligible in and prepared to the current national Street
Touring (ST) class rules are permitted to compete in their respective
Street Prepared classes, with the additional allowance that they
may use any tire which meets the requirements of 15.3 and fits on
the ST-legal wheels and within the ST-legal bodywork.”
So basically, if your car is prepped to ST rules, including the big brakes, you can pop on r comps to an ST legal wheel, and run in your normal SP class. However there is no mixing and matching, so if you have something that is SP legal, but not ST legal with the big brakes, you go to SM.