HP motor ratings don't tell you much about the performance of a pump. I can show you a 1/2 HP pump that produces more flow rate than a 2 HP pump on the same plumbing. So why do you think a 1/2 HP is the right "size"?

FYI, with a two speed pump, low speed will have about 1/2 the flow rate of high speed but only at 1/4th the wattage.