I have a spe26. I'm using an APC 17x8 and getting about 7450rpm.With a Xoar 17x8 , rpm was about 7300rpm.I acquired it from a friend who couldn't get it above 6900rpm on the same prop.I changed the carb to a larger bore, changed the ignition to an RCexl and changed rings to a Bowman. I used an OS160 muffler after some work. I reckon I could probably better rpm with the supplied muffler but I wanted a quieter setup and easier mounting. It was in a 95in candy bomber which probably weighed about 12lbs and flew with authority.
I think a 16x8 or 17x6 would be better for other applications.Al the spe26 in the club ran a 17x8 wood at 6900~7300rpm,definitely not a power house.The compression is pretty light compared to other engines of similar displacement like the CRRC and MVVS thereby the poorer performance.