Back pressure isn't a myth. It's basic physics. The idea of changing the exhaust and header is for more free flowing and adjusting the exhaust resonance.
On a 4 stroke, as the exhaust pressure is going towards the engine, you want it at the same time the exhaust valve is closed. Obviously this is impossible over the whole rev range, but achievable at a certain rev range. One size exhaust might get you a low rev peak power, another might make high rev peak power. The diameter, shape and length of both the header and exhaust system all play effect in the exhaust resonance.