I'm looking to purchase a 97 gt convt. Question is: it's a California car with a slp cat back and bbk cold air. However it has 6 cats on it for emissions. Wondering how much that's robbing and what will be the best replacement? X or h? catted or not? Also being a convt, I'm not wanting obnoxious. Im new the mustang mod game so help is much appreciared, thanks.