When I bought my car it had a high-flow cat hooked to my RMM downpipe. It worked pretty good. I eventually replaced it with a new RMM downpipe with no cat. It made a slightly noticeable difference in performance and it sounded quite a bit better. It was a lot more thoaty at WOT. I don't remember any significant difference in boost levels. I think my max boost went up a bit in 4th gear and up, but it was already going to 20psi anyway. I think my turbos spooled just a hair faster as well. Overall though, the high flow cat worked great.