Got it Saturday.
As others noted, since the RFG operates as eductor, it has to go below the water level, lower than say a flare nozzle, otherwise it will suck in air. For a 24"x24" tank footprint, 1" deeper means an additional 2.5 gallons drains back to the sump. I don't have a big sump, so I have to test to make sure the remaining space can at least take the water from the overflow box.
I'd say RFG is great for a closed loop. For return, just make sure you have room in your sump for additional water coming down from DT when you turn off the return pump.