[Chris in Dallas]

How are you going to insulate your roof/cathedral ceiling? I think you might want to investigate a radiant barrier for the underside of your decking, some low profile baffles to accomodate continuous soffit vents, a ridge vent (if you don't already have one) followed by some spray polyurethane foam for high r-value per inch, so you can put your gyp directly on your rafters.

ETA: Those are 2x6 rafters in there. You'll need a maximum 2" deep baffle, which should run all the way to the peak. You'll only have 3.5" left, so the only thing that's gonna give you a decent r-value will be a really high value poly foam. Check out the kits offered by Tiger Foam. An added benefit of the foam is damping of outside noise for a quieter theater room.