You have been going in the right direction and train of thought! The 10x7/10x8 is probably the best prop for the OS on a muffler. Use an APC or MA Scimitar series prop. Stay away from the standard black MA or even the TopFlite props. Experiment with some 15% fuel. The speed freaks usually go with a smaller diameter prop. Throw on a 9x10 APC. If you really want some more umph then get a pre-tuned pipe setup from MACS. This will give you at least another K RPM.