I would of expected that your low flow results when the pump is running well below its prime to be affected by porting.

You could increase flow by modding the impeller but then a different top would make a lot of difference, then you would probably need a better motor ending up in a completely different pump so its not really worth the effort but would be extreme.

The ideal number of impeller blades is supposedly 6-8 on larger industrial models but it is by no means a hard and fast rule.