Hey Chris,
I have been looking around for the pump that you mentioned and I can't find any info on it anywhere. The largest rio pump that I found is called the Rio3100HP ("High Pressure"). It lists as 900gph at 0" but drops down to 700gph at 3" head. Rios have never gotten very good reviews in terms of reliability I owned a Rio2100 that just crapped out one day and wouldn't start. I'd find out exactly which Rio Danny is using and double check the head-loss. It might still meet your needs and be pretty affordable. I also know a lot of people use submersable Mag pumps. It looks like a Mag9 at 4" head with 3 90degree elbows makes about 650gph after losses. They sometimes come up for sale here for around $100.
What are the dimensions/volume of your sump? I have a feeling that you might be blasting too much water through your sump if you want to create all of your main-tank flow with the return pump. If I were you, I would use the single tube overflow box with a 600+gph pump and then make up the rest of your flow with a second pump. An external pump on a closed loop would be great and would enable you to get your powerheads out of the main. A more affordable solution would be to just use one of the powerheads you already have to supplement the flow from the sump return. Since you would have an overflow box, you would no longer need to have your powerhead at the surface of the water and could lay it on the sand in the back of your tank behind the rocks. All that would be visible is the electrical cord running up the back corner and I bet you wouldn't even notice it. Sorry this post is so long - I should have just got on the phone and called you up
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- Chad