It's just about impossible to make a muffler quiet, small and powerful at the same time. You can choose power and small, quiet and small but not all three in the same muffler. The reason stock mufflers are poor performers is because they are small and quiet.

However, small and loud is not a guarantee of performance. In general, small mufflers with large straight-through, perforated tube baffles (looks like a tube with many holes drilled in it) make the most power and the most noise. An exception to this rule (there may be more) are the popular H-D Screamin' Eagle (and Cycle Shack) small slip-on mufflers which perform very well yet are not straight-through designs. The popular louvered core baffles restrict flow at full throttle & high rpm and reduce power a bit as a result.