Most filter brands state output rates when there is no media "blocking" the water path. My JBL e701 states 700lph, but the output when media in the baskets is 400lph at best (probably closer to 300-350lph).
All you can do to maximise flow is keep the impeller and its cavity clean of "gunk" that builds up over time, using removed tank water and a cotton bud. Strain the filter media baskets every now and again to remove similar "gunk." How often you do this depends upon factors such as stocking, how much you feed your fish etc.