i talked to a tin knocker the other day about this, hear goes something, he said that it does matter to a degree, when you push air through a completey round pipe it flows like water smooth, there are currents in it but they swirl through the tube, but if you distort the shape say make it rectangular the flow will change cause air likes to swirl, but you can compensate for it by changing the overall cross sectional area of the pipe/duct. if there are kinks, the create turbuleunce this causees drag or like air pockets, behind the kink where they make dead spots. take your garden hose turn it on, let the water flow you do not hear the water, kink it slightly or all the way over to exagarate it but you can hear it going like shhhhhhrthrhrh that is the turbulence but air is not as dense as water so the drag is less and it only really matters when you are trying to push large amounts of air through a system effeciently do you really start to notice. so go from 2.25 inch pipe up to 3.75 put all the kinks in it you want and it does not matter there is no real back pressure to deal with then/ just kidding about the last sentence.

 

I see you got the importance of mandrel bends and I doubt that muffler makes a noticeable difference and looks terrible doing it.

 

It depends on the angle of the bend. A mandrel bend 90 degree elbow flows MUCH better than a 90 degree crush bend, but there's very little difference in a 10 degree bend. If you are buying pre-bent pipe sections to build your own exhaust, go with mandrel. If someone wants to custom bend for you (common when using stainless or aluminized steel is more important to you than 1.5 hp) than go with whatever they can do.