[Blom]

From what I was able to find online the 1 1/2 PVC drain line will be able to handle 2100 gph based on a gravity feed. Depending on how high your water column is above the intake you will get some added flow due to the weight of the water, which in turn creates pressure. But it will be a modest gain at best. Im not sure which Dart pump your running (Ill assume the 1/8HP due to its 3600gph at 0' Head) or the hight the water has to be pushed to (Again Ill assume between 4-6 feet). With that pump at 4feet you will get 2800gph roughly, and at 6feet it will reduce to 2400gph. Based on those numbers your right, you would have to throttle back the pump a bit. Im not sure but I think that would shorten the life of your pump (due to the added resistance). But Im not sure about that.