In the OS web site they recomend smaller prop (Max. 15x6). I am wondering if the 16x8 is too much load for the engine using standard muffler. Probably good with tuned pipe. I don't have this particular engine so I don't know. Check here:

http://www.osengines.com/engines/osmg0650.html

In regard the fuel. I am almost sure that the lost of power with 5% will be too much for this application.

VB