Yeah but that's without media and plumbing. I'm not very familiar with the fluval canisters but, my xp3 rated at 350gph actually flows about 180 gph filled. I'd bet the fluvals are pretty close to that as well.
So figure around 180-200 gph per 404 and 600gph for the fx5. Turnover is right around 5x per hour which should be good
Sumps arent really that hard, they just take more research and planning. Canister is basically plug and play. A sump takes a little more thought to how you want it to function and it's very rare that two sumps are setup exactly the same way. It's a great form of filtration but multiple canisters can work just as well
I have a wet dry built into my 90 and am setting up a 75 gallon sump on my new 300 which hopefully will be here by the beginning of July. Ive got everything fir the new sump in my garage driving my wife nuts. I also use numerous canisters and hobs. I've got filters eveywhere! Lol
It is possible to do a sump without drilling your tank. Like the poster above, just get a eshopps hang on the back overflow OR make your own overflow out of pvc (pictures are in the diy section). Return line, just use pvc