For the sake of comparison, I was running a pair of Iwaki 40XLRT return pimps on an Ocenaic 200 with 2x 1" drains. I do not know what the flow rate was, but the drains could not handle the full output from the pair of Iwaki pumps. FWIW, the return lines from each of the Iwaki pumps included 5.5' vertical head and 4-5' of horizontal run, with 1 90 degree bend.
I also reached the same conclusion about using an Ampmaster 3000 as a return pump for my new tank. It is not designed to handle very much backpressure. That is why I opted for a Dolphin 4500. This is a high rpm pump that is much better suited as a return pump. I hope to get at least 2400 gph out of this pump eventhough I will have 6' of vertical head and will be splitting the single 1.5" return line into 8x separate 0.75" return lines, each with a 90 degree bend. I will have 2x 1.5" drain bulkheads to handle the flow to the sump.
By the way, I still plan to use the Iwaki pumps in the new set-up, but on separate closed loops, each connected to a 1" Sea Swirl and a 1.5" intake bulkhead.