The black box muffler is restrictive, It is meant to be compact, not competitive. You may gain a few hundred RPM with the bisson or the MVVS #3271, ending up in the better part of the 6000's. Any muffler made to shape of the planes nose, and with a volume over 15 times engine displacement would do as well.

For your information,

A Moki 30 turned a 20x8 prop at 5800 rpm, and so did the Supertigre 2300, both with the Krumscheid "banana" muffler. Anything above 6000 with that prop is very usable indeed.