Occlusion culling for flight sim

I'm getting the sense that occlusion culling would not be possible for a flight sim type game.

Maybe my understanding is flawed but it appears occlusion culling produces cells intended for a character to stay on the ground.

Am I understanding this correctly?

Any general tips for improving the performance of a flight sim type game targeted for iPhone?

I have non-pro Unity and non-Pro iPhone and was starting to think that upgrading to pro would help game performance but I see no real benefit for flight sim type game ...