Remember that the length of pipe and it's vertical lift will seriously effect pump capacity so you should consider this when siting the pump.
A 1500 gph Rule will pump 750 gph if the vertical run is 6'. For 8' the capacity drops to 500 gph. Also, every 10' of horizontal equals around 1.5' of vertical head.
If your 1500 gph pump is connected to 10' of horizontal pipe and then it pumps vertically for 6' your total head is 7.5'. With standard 1.25" pipe the pump will move a maximum of 600 gph.
Capacity on these electric submersible bilge pumps is greatly influenced by pressure head and any other resistance such as that from a non-return valve.