I have a similar situation but a different strategy. I live in an apartment in which I am trying to reduce airborne and impact noise from above. I think I've read every article on the internet about how best to accomplish this goal, and virtually all of the "best solutions" involve tearing down the existing drywall ceiling which for me is not an option. So here is what I am considering:
1. Fill the cavities between the existing ceiling rafters by blowing cellulose insulation above the existing drywall ceiling
2. Apply isolation clips (Whisper, isoTRAX, etc.) to the existing drywall ceiling (please read on, I realize this is considered bad practice)
3. Apply hat channel to the isolation clips
4. Apply 1" thick rock wool bats between the hat channel rails. These bats will fill the space between the existing drywall ceiling and the new layer of drywall
5. Apply new layer of 5/8" drywall (or QuietRock) to the hat channel rails
I realize that affixing isolation clips to an existing drywall ceiling is considered bad practice because it can create a mass-air-mass resonance chamber between the two layers of drywall that actually can worsen the noise. However, in step 4, I would sandwich a 1" layer of rock wool (or fiberglass) bats between the two layers of drywall to fill the void and absorb the sound waves traveling between the existing ceiling and new layer of drywall. So the final structure would have 10" of cellulose insulation between the rafters, the existing drywall ceiling, 1" of rock wool, and a new layer of 5/8" drywall (or QuietRock) affixed to isolation clips. In the end, I won't achieve full STC reduction, but I assume this would provide some additional isolation of both higher-frequency and lower frequency airborne and impact noise. Please let me know if you think this strategy has any merit.