It's a common kid conception you want back pressure in actual fact it's exhaust gas velocity you want back pressure is just a by product if this. I don't realy interest myself in the n/a aspect of tuning but for example on a turbo car you want to try maximise the exhaust gas velocity by running smaller diameter exhaust manifold runners to spool up the turbo as fast ad possible and then run the least restrictive exhaust you can. Althought I could be wrong but that's my understanding