Don't be confused by the oft repeated "need for back pressure" to increase horse power. If that were the case, you would never see "open headers" on a race car.
The length of a header collector, in some cases, has an optimum length. One way to find it is to look carefully at the scorch marks or some other method that shows the temperature of the exiting exhaust gases. Then start cutting off the collector length until it just reaches the desired point, right at the highest temp exit point.
A two cycle engine certainly has a required "back pressure" point because of the nature of a two cycle. A properly tuned "expansion chamber" uses the escaping momentum of the exhaust to help "pull in" the fresh intake charge. That works best at near max rpm range. To some extent, this also applies to a four cycle, but not nearly as much and the "expansion chamber" becomes the "collector" on the end of the headers. No mufflers, in either case will add anything to the scavenging affect.