I have found, at least with the engines I have, that a 1" diameter change doesn't affect rpm too much, whereas the pitch can drastically affect the rpm of the engine. 7" pitch to 6" pitch can be as high as 1500rpm depending on the engine, but from a 9" dia. to a 10" dia. only affects rpm by maybe 200-300rpm.

I suspected the load was too high for the engines, so dropping some pitch while maintaining the diameter should give more throttle response, and perhaps give more speed because of the higher rpm. Unless you drop from a 7" pitch to a 4" pitch that is..