No problem, they run the same either way...The instruction manual says to use a prop thet will turn 7000 t 8500 while flying...Many customers using many props have found the Mejzlik 18-6 to give the best performance...The stock muffler cut 800 rpm from a less restrictive one...
I don't know what it turns in the air with that prop, but the peak torque happens at 9000 rpm, the HP is at 12,000...Why run a prop that won't let the engine run it it's best torque range ?