The thing you want to do is to optimize your exhuast airflow velocity.
It's not that you need backpressure. Those ppl that put in bigger piping do lose torque b/c the velocity of the airflow is slower then stock in the low rpms. So they go complain about "I need backpressure!" blah blah.. But in the higher rpms, you gain more hp then before.
Poohboi's example is good at showing this also.