That is pretty much what I have and my fish are happy. The FX5 is on one end of the 300 and on the other I have an overflow that does about 1300 gallons an hour.
I have a 300 with an fx5 by itself and seems to do just fine. Personally I feel that I'm under filtered, but has been running with my stock just fine for several months now.
no is the short answer. unless this iverflow was ahole drilled in the middle of the tank./ basically when air starts to get in to the caniuster bad things happen a shirt while later
I have 2 fx-5's and a Mag 18 in my sump on my 300....I felt i was under filtered so I added another one..I want to add allot of fish to my tank and I feel over-filtration is never bad...