1. If I were to set out to make an immersive film, I'd block it so that at any time you needn't be aware of the full 360° area you're in. This gives you room to cheat a camera, focus puller etc.
2. Render time gets cheaper all the time, and it depends what you're doing of course. Rendering the rear wall of a room (to hide a camera crew etc.) would be child's play compared to creating the floating islands on Pandora with a vast array of flying beasties.
All about how you choose to apply the technology.