Better oxygen exchange

[Jeff_]

There are a couple things that could increase o2 concentration.
1. Skimmer- pretty much any skimmer will increase the o2 concentration. Look for the skimmer that has the highest air intake?
2. Air pump- This would probably cause a ton of salt creep, but will increase o2
3. As you said a larger sump means more area of gas exchange and any kind of surface agitation would also increase the o2 content.
4. Lastly, a refugium with 24 hr lighting. The photosynthesis would add o2 in the water.
HTH