I'm also waiting to scrub off my tires before replacing them with a matching set on all four corners, but FWIW, I've run mismatched F/R street tires at my six most recent track schools, and the M3 handled great. Would it have handled even better with matching F/R tires? Maybe, I can't say for sure. But to the OP's original question, I don't think there's necessarily any harm in running PS2/PSS. It's not like running winter tires in the front and r-compounds in the back.