Originally Posted by Jayhovah

I am, by no means, an expert in this stuff.. but I always thought the benefit of back-pressure wasn't really the pressure itself, but the increase in exhaust gas velocity in the lower rev range (which you want for scavenging purposes)... this is why you lose low-end torque if you go way oversized on your exhaust pipes. I think keeping the velocity in a good place is important for both NA and SC, but you have to factor in the big increase in exhaust volume when you go SC.

No matter how you slice it, the OEM cats are an enormous restriction and are not beneficial to making power. Otherwise, few people would buy HFCs or test pipes.

You might see more boost with OEM cats, that is only because flow through the motor is being restricted... without them, boost pressure likely reduces and power will absolutely increase. Often times this is a misunderstood concept and you will see people wonder why their car makes less power at the same boost pressure compared to another with a different setup.