5.6 Multi-level navmeshes

Does the new navmesh system support multi-level navigation? In specific I'm going to use a 2k mesh terrain that has holes in it with a cave system underneath that mesh terrain, all in one scene.

What's the performance like for a 2k navmesh? Especially with updating it at runtime as things are created/destroyed? I have dynamically placed trees that grow/die/respawn at runtime.

Also wondering if it can handle flying/swimming creatures.