My 10 cents: Just put up another layer of drywall... when properly installed this will provide a higher stc and be a lot cheaper than the barrier materials. There are good ways to use these materials, but sandwiched inbetween drywall is not the best application. At that point, it's not a 'limp barrier' and it's mass at 1lb/SF is not as significant as another layer of drywall at approx 2.5lbs/ SF..
my 10 cents.
FM