The general rule is 1” is supposed to equal about 600
gph but depending on the brand and prefilter/sponge you have it may vary. Does your overflow drain straight\curved down into your sump or do you have a couple of 90 degree bends? Is air trapped in the overflow? How long have you been using it and is the drain clear or need to be cleaned?
With it splitting off to separate sides it should only be putting out about 400
gph so I’d say something is off on your overflow if you have to scale that back to half. (I assume with a ball valve??)
I use the
CPR CS102 overflow and it has no trouble keeping up with the mag 9.5. I also like that they have a
tech support forum.