Ours is probably about a STC of 40 for the walls and maybe 46-48 for the ceiling based on extrapolation of additional batting thickness on STC. We had considered the staggered studs method which would have added maybe 5 to 7 more STC but really, i doubt anyone would notice substantially in the upstairs rooms. No one cares much about sound immediately outside of the HT room itself and a 30dB drop in SPL has been measured just by closing the HT doors. In the meantime, we would have decreased the media room size which was a larger concern.
If we ever decided to do more in the future, adding a second gypsum layer would add 5 STC and not substantially change much else in the room, although the pain of going through more mudding and sanding wouldn't be worth it IMO especially considering we have ducting in the insulated walls which is likely the weakest link in the acoustics that escape the room.
