As stated before Backpressure is not good But Velocity is very important if a combo is well tuned more air is pulled into the cylinders with scavenging using headers than the pistons pull in , this is where the hot air leaving at high speed creates a vacuum behind it pulling air into the cylinders with the exhaust & intake valves are both open during the overlap in the cam , at high rpm this can actually create mild supercharging where the cylinder can be filled to more than 100% volume . Manifolds do not have runners long enough to effectively create scavenging .
If the exhaust is too large the speed is lost inside the exhaust , too small & it becomes restrictive where the speed needs to be excessive & causes the engine to work harder to push the exhaust out so it is a balance .
I read an interesting book on exhaust science year ago & have forgotten too much , wish I still had the book !! Maybe it is on Ebay ?
As dumb as it sounds as the exhaust cools it needs less pipe diameter to maintain the same flow so the factory had it right where the head pipes were generally larger with small 2" pipe at the tips , this has been proven to increase power also everything after the muffler has very little effect at all . I have never seen an aftermarket system where the pipe diameter reduces further back I would imagine most would view this as restrictive therefore creating a demand to maintain pipe diameter all the way to the tips . This has been the reasoning behind new stepped tube headers where the pipe diameter is much larger right @ the head & the pipe size is reduced 8" or so away from the heads , these are also proven to increase power .
Headers are also tuned for rpm , very high rpm engines need shorter primary tubes as the next exhaust pulse is very close behind , low rpm torque engines benefit from much longer tube headers as the pulses are farther apart .
Straight trough mufflers while seemingly should have very little resistance to flow were actually shown to be very inefficient as well.