It'd be worth measuring the impeller diameter and blade height you have Bruce, also the diameter of the fan inlet and exhaust. That would at least verify whether or not impeller size is an issue, or if it's something else. For what it's worth a bigger motor doesn't deliver higher CFM unless the impeller is big enough to load it/move the air, and the system is free flowing enough that the flow comes at a pressure drop the impeller is capable of at the motor RPM.

ian