i mean you can move a lot of water but you can't remove waste that's not there. your biofilteration especially is dependant on fish waste. the amount of food for your bb colony is the limiting factor to its size. No matter how much surface area your media has and no matter how many GPH you pump through it it wont remove any thing from your water if its not being produced by your fish.
i think X amount of filters for X amount of gallons means nothing if bioload is not considered. These numbers are an indicator of your filter's capacity to work and not what is actually going on in your filter.
instead of gallons...biomass would be a better number to compare to your battery of filters.
i have a 135 with a fluval 405 (340gph) and 4 aqua clear 70's (400gph each)hooked to under gravel so 1940 gph- so it turns all the water over 14 times an hour. not too bad. i want an fx5 but am too broke right now.