[Geoff]

Just my opinion but to go to the amount of trouble to double wall everything ,you better go all out with duct work and electrical outlets,etc...
Not worth the trouble if you ask me and like you said ,it eats into the dimensions and budget.
Also keep in mind that a projector mounted on the ceiling with a screen that size will probably put viewers heads right at projector height as they walk by.

 

[Geoff]

as I mentioned with regards the position of the projector, try to position it directly above the 1st row of viewers and keep the cables as high as possible. Those on the riser will be banging their heads all night if you don't.