Even with a Slimline pitts muffler I'm getting 8700 RPM's with the 17x8 and that pulls this heavy Pitts around very well. I did have mine lightened from 53 oz to 40 oz with an EI conversion but you should be getting good performance still. If you have the clearance you should try a good 18x6. Should be able to get 17 lbs of thrust with that prop. Are you looking for speed or torque?