The vaulted ceiling pieces I cut a few days ago fit perfectly. They strengthened and tied tougher the top of the ceiling joists where they attached to the ridge beam and also gently wrapped the curved ceiling over the top of the room. I kept the cross strapping to a minimum (for now) to allow room for the spray foam guys to do their work.
In the lower ceiling of the room the arched corner pieces were screwed in place to round the corners slightly. Headroom (both visual and actual) was not compromised in the process.