From what I understand about sump style filtration systems (though I'm a newbie, you may want to take what I say with a grain of salt
), you shouldn't have too much flow through the sump. I understand that you need lots of circulation in the tank (which is why people use closed loops and powerheads) but that you only need as much flow through the sump as is needed by the filtration equipment (ie: 500 gph for the skimmer feed, + 90 gph for some reactor, = 590 gph). But this will depend on factors like whether you run a refugium in sump, and how much flow that will need to be efficient, etc.