Man there are going to be some that will disagree with my preference but my theory ( as far as water proccessing in the sump goes) is the more water you can flow from the display to the skimmer in the sump, the more crap the skimmer will have available to skim. I would say your 900gph is right on...depending on head loss too.
I like to use the extremes for examples so, lets say you only flow 10gph through your sump. Your skimmer is going to have alot of contact time with the sump water and you will be returning some very well skimmed water to the display...but at rate of only 10gph. Meanwhile all the fishcrap is building in the display and is only being diluted with 10% ( at 100g display) per hour with skimmed water. But when you flow your 900gph it will be sending all that fishcrap down to the sump faster, where the skimmer will have a shot at much more of it, more often.
I'm sure there is some real good math out there to tell you the happy medium where more is just a waste of time. I subscribe to the 6-10 times per hour club. what type of pump? some can have some real headloss!