I don't know much about sumps, but I would make it so the first part filters out the particulates and you can clean that regularly. I don't know why Bio Balls would be a nitrate factory except for the fact that you do want bacteria colonising on them which convert waste to nitrites and then to nitrates. You don't want them getting all full of gunk so they should come after the particulate filtering material.
I was dumpster diving again today and this aquarium store uses water polishing filters as well as large floss bags for particulates. I've also seen Bio Balls used in aquarium store filtration systems. Just food for thought. I don't really know how they do it as I haven't taken the time to investigate. I actually like the idea of using pond filters which come in various sizes, and I used to have a hot tub filter I was going to use for a pond but never got around to that.
The hot tub filter seemed like it would be a fantastic high volume filter. I was going to have a smaller particulate filter come first which could be cleaned regularly, and then use the hot tub filter as a massive biological filter.
There are people on this site who are thinking of large volume filtration, so I imagine they'll put there ideas in this thread too.