I believe the Fluval FX5 is rated around 900gph, although thats subjective too since most of those filters are tested w/ no media (don't ask me why?) So depending how you pack it, subtract 25/35%.
Sounds like you're gonna end up w/ a modular or multi-can setup anyway!!
That $350 will be $500 or better before you know it. (more cans, back-up pump, xtra fittings ect.) Don't get me wrong. This is a good thing especially if you're gonna stock heavy. There's no such thing as too much filtration! Unfortunately the more intricate the set-up, the more you have to do. You can extend the time by oversizing (another good thing) but you're gonna have to get in there. The results however will make the cost & the work more than worth it!! Good Luck, "T"
OK. You seem convinced so I'll back out. (after all it's your 'chedda'!!
) Do re-think that back-up pump though. The 2yr. warranty is good but that just means they'll replace it in the event of failure during that time frame. In the mean time, no pump, no filtration. No filtration, well you know.... Think of it as insurance, or like a spare tire. The car will run just fine w/out it but there's nothing like opening that trunk & pulling out that 'ole' spare if you've got a flat. It can be a LIFESAVER!! 
Good Luck, "T"
