I don't know brosef... I think I'd just keep things simple.
Grab your self a Phosban 150 reactor, throw some biopellets in there, run it off your return pump, and enjoy your fuzzy sticks.
Not even worth making your sump into a sump/refugium because unless the refugium is a substantial size so it'll really put a "dent" in your nitrates, theres no point in attempting it.
I disagree with this just a bit, In a SPS ULNS the fuge is more important not for nutrient export so much thought it will help a bit but for having a safe place for pods and bugs and all the other critters to grow.
Now the skimmer that pulls the wet, does that have more water at the top as to having the water level at the bottom of the skimmer head.