About the cubes tunneling through terrain, that's nothing I can fix. That's an internal Bullet thing. It's only because a cube has 4 points. You won't encounter that with any other shape that can do accurate measuring when it has impacted something at high speed, because they have more points (vertices) to check collision with. If it has tunneled more than 50% into a mesh, it will be shot right through. The cube can't calculate that for the whole center of it, so tunneling will occur often at high speeds.
You can also set the CCD values to avoid tunneling if you happen to encounter this with a tri-mesh or other shapes. Boxes are just too lightly detailed for being fast moving objects when they collide at strange angles and/or get an edge collision.
CCD doesn't really have an effect on cubes, either.
btw: a subdivided cube wouldn't have this problem, especially if you allowed CCD on it.