I've got a new to me Moki 2.1 that can only swing a Xoar 20/8 at 6900/7000 RPM. It's got a Perry pump/reg that supplies enough fuel to run slobbery rich if needed. Large fuel tube and clunk. It is side mounted and has a Bison Pitts Muffler. OS F plug running on Omega 5% with a little extra Caster added. The engine runs great otherwise.

Is the Xoar 20/8 a high load prop for this engine? I honestly expected 7500+ RPM, maybe closer to 8K. The benchmark prop for the 2.1 is supposed to be the Moki 20/10 @ 7500. My altitude above sea level is 900'.

I've heard that the Bison mufflers can restrict the exhaust a bit without trimming 1/4" off the tubes. For those in the know, how much more RPM should I expect after trimming the tubes?
BB