Hmmm. Exhaust gas travels from the powerhead, out the exhaust ports, down the lower unit, and out from the prop nacelle, either through or over the prop.
Personally, I don't think the hub size on the prop is going to make much of a difference in exhaust flow. Too much other resistance. It also won't create less drag, as the nacelle leads it, and has a fixed cross sectional area.
I'm not real clear how a smaller hub diameter will increase exhaust flow, either. Seems backwards. To get more flow, you want a larger opening. Smaller will cause more backpressure, higher exhaust velocities, and perhaps *less* flow.
OTOH, I could very well be wrong.