So, there is a difference when it comes to building the roof. The problem I ran into when building my roof wasn't so much that I was concerned about the completed panel sagging, it was more about the build sequence. I was using 5mm okoume ply for my inner skins and ceiling, and the weight of the ceiling panel by itself was enough to cause that to sag. I'm pretty sure if I had built a form or bought lumber for temporary props, I could have built my roof up and after it was all glued with the arch it would have supported itself fine... but (pause for affect) the cost of that extra lumber and the effort to build that tooling would have been at least as great as just gluing traditional spars to the top of the ceiling panel before installing it. By using mostly select cedar and some poplar for the spars the weight penalty was minimal.
I agree, for your wider span some form of arched rib (foam) or embedded spar (wood or FG) is probably a good way to prevent sagging.