I think the wheels and Tein Flex would put you in STS2, but the butterfly brace will put you in Street Mod 2, SM2. Talk to the other STS2 drivers, they may not mind the brace, at least until you start getting close or beat them. If you are on street tires, you could run TSM2. ST* are not eligible for Tire class.
Edit: After looking at another site, I think you are right about the butterfly brace putting your car in Prepared, because of the cross bracing.