i didn't notice any significant power increase when I put O/R pipe on it, but when I put OEM pipe back on to get inspection done year later, I did noticed that OEM pipe have noticebly more low end torque. mainly due to 4 cats create more back pressure, which helps engine to make more low end grant.
Little loss, but thats price I'll more then happy to pay for significantly increased exhaust flow above 3000+RPM. and the sound of prochamber
Only way to get low end torque back is to put more back pressure(i.e. put cat on the pipe), that would defeat the propouse of O/R pipe
Another way to get more torque would be stroke it