[Fast Eddie]

Actually, you do want some back pressure. Since there is typically overlap between the time the exhaust valves close fully and the intake valves opening, if the flow out the exhaust is too fast you will pull the intake air and fuel out the exhaust side. That hurts performance, since the whole goal is to have as much stuff blow up in the cylinder as possible.

On turbos the turbine provides the back pressure so everything after that can be as free as a bird, without hurting performance. You also want to have it flowing as fast as possible from there to keep the temp down.

I've got charts somewhere at home (from when I cared) showing the difference from stock to header->larger piping->high-flow cat->performance muff. And there is a reduction in TQ.