Plaster all done (we're doing our PCI tomorrow actually, so potentially keys this week).
We got them to put their highest available option (R3.1 110mm soundbreak accoustic glasswool batts) in the ceiling/walls, so just matter of it actually being done properly (never know with project builders till we're in).
Yeh, the 127" scope was about 100" 16:9 in my calculations as well. Going into this, I was expecting 100" to 110" at the maximum for a 16:9 screen, however with the theatre builder, there was a price option for a 120" 16:9 that was "doable" in the room.