The sphere is by design. An axis-locked sphere allows the craft to ‘hover’ a set distance from the ground, and also makes our collisions more predictable. For example, one game mode includes hitting other balls into a goal… a sphere vs sphere collision is more predictable to the player than detailed mesh vs sphere.
I do not think the sphere is the problem. I cannot stress enough that the sphere solution works great with imported static meshes despite the mesh being more complex (higher resolution) than my testing Landscape.
I have seen similar problems in the past when I was using the Bullet physics engine described at the following link:
http://www.bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=4603
Basically collisions were happening with internal edges, which is exactly what it feels like with Landscape. We fixed the problem in my previous game by enabling this btInternalEdgeUtility. I wonder if something similar exists for PhysX? In any case, static meshes ARE currently handled correctly… it is only Landscape that exhibits this bug.