The catback does become an issue at higher power numbers. I was knocking like a ***** on 18 PSI of boost on my turbo setup. Swapped to ZZP catback (which honestly, I thought sounded good but I was turbo so everything I know is a lie) and all the knock was gone. I then turned it up to 26 psi and KABOOM wen't the transmission, but we dont talk about that