RE: SYSSA 30CC GAS MADE IN USA
2 things come to mind.
Good sport performance generally requires pretty good vertical performance? That generally requires props with no more than 10" of pitch max. Generally less.
This engine really likes to run in it's upper rpm ranges to make max thrust. Maybe turning something in the neighborhood of 8500 - 8800 on the ground (once broken in a little)?
I run an APC 18x6W on mine, which has worked out pretty well - but it's 13+lbs. too. Listening to it unload in the air is a LOT like hearing an old .60 size pattern plane fly! Todd special ordered those 18.5x5.5 props for 3D work on a 10lb plane. That should give you an idea regarding props that work well?
Your 18x8 should work ok on a fairly light plane, especially a cleaner design. The others have way more pitch than anything I would ever use - unless you were talking military style flying. -Al