What I was suggesting was to put an extra pump in the the wet/dry sump and plumb it into the refugium, then run the overflow box from the refugium back the the wet/dry sump, that way there is no risk of overflowing the refugium b/c the overflow box will take care of that. Also this will require smaller pump.
For my personal choice I would get an extra pump and put it in the wet/dry sump and plumb it into the main tank, if the main tank can handle the extra flow that is. Then I would plumb a T pipe in before the the sump, run one end of the T pipe into the sump and the other into the refugium and plumb a ball valve into the pipe that goes into the refugium(to control the water flow into the refugium). Then put the overflow box on the refugium and plumb it back into the wet/dry sump.
In any case to ensure no overflowing is to occur it is essential that you have some sort of overfow system implymented. Using pumps to try to balance water flow is not only risky it requires a lot of work to ensure that there is always a balance, and IME and IMO deffinately not the way to go.