Maybe the muffler is holding them back. It could be piped timing as well if the exh timing seems anywhere near 170 degrees. Then it would need a pipe. Otherwise, yes it does seem somewhat average for a motor of that size. How are the revs with no muffler? The old .29 motors were pulling a 7 or 8" prop with a 10" pitch at 20,000 rpm. That was with very high nitro. Now the pitch is lower because of the 10% fuel mandated. I think 24,000 rpm is what they are propped at on the .21 size speed planes. Nice video .