Aloha Seawell,
Thanks for updating your profile.
Isolation is accomplished using the mass-air-mass principle, however adding "another layer of insulation and drywall will give you "three leaf" construction, which is what you want to AVOID if possible. Since you already have your inside and outside walls insulated with rockwool, what you could do is:
Interior
1) remove any molding around the floors, ceiling, windows, etc. and check if there are any gaps where the drywall ends. If so, (more than likely ) caulk all the gaps with acoustical caulk (not the same as regular caulk), using a backing rod first if the gaps are too big.
2) lay 2nd layer of drywall, overlapping seams (if visible) of the first layer, again, caulking all gaps.
3) lay 3rd layer of drywall, overlapping seams and caulking all gaps.
Exterior
Depending on your budget and how much effort and $$$ you have/want to put into achieving isolation, the best case scenario would be to remove siding, add 1-2 layers of plywood overlapping seams and caulking all gaps (between layers if more than one), then reinstalling siding.
You might also want to consider whatever window and ceiling issues may exist.
Would something like Auralex Resilient Channel help as well?
Not unless you were planning to remove the first layer of drywall, then mounting RC, and then 2-3 layers of drywall, caulking between layers.
On the outside of the studio on the wall facing my neighbors house is vinyl siding if that makes any difference.
Not much difference isolation wise (doesn't help.)
Here's a few links for some great info:
http://www.saecollege.de/reference_material/index.html
http://www.johnlsayers.com/HR/index1.htm
http://www.johnlsayers.com/phpBB2/viewtopic.php?t=2125
Hope this helps.
Looking forward to your build!
Aloha
