Now I'm no expert, but based on the hours of research I did to find out everything I could before doing this mod, I would assume it has little to no affect on them. I haven't read or found any posts on Catalyst failure with exhaust flaps to blame. After all, the stock setup bypasses the muffler above 3500 Rpm. Was that to prolong Cat life?.... no, it was to improve bottom end torque. Plus, look at all the cars that have aftermarket exhaust which totally eliminates the flap system. Surely by now if it caused problems you'd see evidence of it but all I see is Aftermarket Exhaust Promotion !!