6+ lbs/square foot? Wow. Heavy stuff; over 190 lbs per 4×8 sheet. Glad I didn’t have to install it.
You have an unusual situation: Almost none of the in-room sound escapes, even without absorbers inside. So there is very little baseline data to form a comparison. There are on-line calculators around that can estimate (untreated) room response based on dimensions and wall/ceiling construction materials, though at this point it would be an academic exercise since you already have the stuff installed. If it were me, I’d just accept the “win”.