cr22: I'm not doubting your expertise and admit i haven't used Trace for residential yet. It may also depend on what method you use (RTS etc.), what design conditions you chose (99%, or 99.6% etc.) and what indoor conditions. Just with these three variables results in the same software can vary widely.
If you undersize, you reduce the airflow, but keep DAT the same? If you just reduce DX size,you get warmer DAT, so you dehumidify even less. for example, if you calculate you need 800 cfm at 55°F, you then reduce to 700 cfm / 55°F? i would think you don't reduce by going to 800 cfm / 60°F DAT?
I think I have a hard time understanding what the basic difference between commercial and residential is. Heat transfer through envelope is the same at given assembly, heat and moisture capacitance is the same, ventilation load depends on ventilation rates, solar load on glazing and orientation, internal loads like plug load etc. are treated the same. the bid difference is how frequent the loads occur. For an office i can assume the plug, lighting and people load is constant fro the scheduled time. for residential it is not. And in a 2-people residence, those two people may be in one room only, for short time. So assumption have to be made for that, that are different from commercial. I would think Manual J has some ideas for those assumptions, I couldn't really find a free publication that details manual J more. If nothing else, residential loads are a very simplified version (no multi-zone, we don't deal with complex equipment, very little ventilation etc.). so with all the right parameters and assumptions i think Trace could do the same. But again, i don't want to discredit your experience, especially since i haven't tried it out myself. what software do you use for Manual J calcs?
JJ: what are the actual temperature setpoints, and what humidity do they have? you say too humid, I don't know what that means. 50%, 70%? Do they have permanent ventilation, or does it cycle with the AC? Are the units tight construction, or have a lot of infiltration? i think there may be more to the problem than just unit sizing.
another problem I've seen with residential units is they discharge at ~60°F, while commercial AC discharge down to ~50°F or even below. So your dehumidification decreases dramatically. The AC contractor may not have charged the units properly. Just an idea, i recently observed that on a building to be part of the problem.
So you should provide more info, or field-survey what is going on. sizing of the units may be one problem, thee may be many others.