From what I understand you need mass to attenuate lower frequencies that your sub puts out. The drywall is the really important part. So important that 2 layers is recommended. Not to seem like a freak but if it were me, I would put 3 layers up there. First 5/8", then 1/2" and finally 3/8". I understand that they block out different frequencies.
I'm not sure what your wood paneling is like but I would be concerned about how reflective it is. Nothing wrong with taking your time and gathering info.
From another thread I got the idea that your ceiling was 8'. I like smooth finishes on drywall and painting it white would give it the appearance of being more open.
Edit: Filling the void completely with insulation without opening the ceiling sounds unlikely, kind of like building a sea worthy ship in a bottle.