Theoretical:

You can test this in game. Stand somewhere near a mountain (Riverwood is good), open the console and type "TFC", then fly straight up so you can see beyond the mountain. Occlusion planes prevent the engine from drawing land geometry, but do nothing to stop it from drawing trees. There's http://www.nexusmods.com/skyrim/mods/55240/? working to improve performance by placing occlusion planes over water, to only draw everything below water level when the player is underwater, but again, it only prevents the land from being drawn, and still renders grasses and other small objects.

There's a good chance this is impossible to do, but that got me thinking there is HUGE potential for a mod that improves the efficiency of occlusion planes in Skyrim. Has anyone with the know-how looked into this?