It ain't the prop - the Funtana 90 on that engine with a 18x6 runs in 9000 10000 range -on a stock pipe setup from Horizon.
The in cowl mufflers run OK but power is waaaaaaaay off compared to the stock factory pipe -or the Skorepa pipe pictured here
the this thing is ballistic compared to the glow engined setups -we let others using YS setups fly it - they were in agreement.
I put in a lot of time with both pipes- power was same but th carbon fiber setup is much more quiet. Also I flew the other 26 I had on stock Evolution Pitts muffler - good power and would hovr and recover this Velox. but no comparison