Technically if you have the ASR brace you are at least in Street Mod. HOWEVER, there are a good number of cars running all over this country with that same brace (whether it's the Comptech piece or the ASR piece) that are running in ST/SP. The cars are illegal, but they are still running in that class.
If you are running locally, not divisionals or nationals, I would be very surprised if anybody protested you over a rear subframe brace. If they did, you would lose, but I have never in almost 10 years of autocrossing and doing open track days seen a protest at a local regional event.