mr limpet, ur suggestion of using an FX5 with a 304 is exactly what i have on my show tank after recently adding the FX5 to the 304 that had already been in service for a number of years
being the workhorse, the FX5 is chock-full of bio filtration, with scrubbies in the top & bottom trays and the middle tray is full (most of a 2lt container) of Seachem Matrix, along with the original mechanical foam inserts
given the FX5's pimary function is to provide maximum bio filtration, I don't use any polishing pads or similar so it won't clog too quickly & require more regular maintenance than it would otherwise, in doing so it does fall a little short on water polishing qualities, this is where the 304 comes in
the 304 picks up the slack in the water polishing dept, while the top tray contains Aqua One Advance bio media the bottom two trays are full with filter wool. I've got the filter wool on a cycle where I only ever replace half of it at once during maintenance to ensure minimum impact on the bio balance while maintaining constant crystal clear water
if i ever have to run some chemical media in the tank such as charcol or whatever it can go in the place of some of the wool without any disruption
as for the waterflow from the FX5, that was an issue for me - or more for the fish - initially. I'd used a spraybar on the 304 for a long time (& still do now) and used to use an internal powerhead to provide some current within the tank but it was on a timer and ran for only a couple of hours each day - now with the brute strength of the FX5 blowing 24/7 it took the fish about a week to gradually get used to it - but since they are quite happy and couldn't care less - in fact they can often be seen having fun swimming against the current - needless to say I don't have the powerhead in there any more
I've often read within this forum and others where people say there's no such thing as too much filtration (incl. disciple in this thread too) and after adding the FX5 to my show tank and seeing the difference in the water quality (nitrates are lower than ever & algae is non existent whereas before it would need cleaning off the glass every week) I can confidently agree and recommend it