I suppose this may help...
I and others have had the problem of falling through the floor/terrain when we
up-graded to the A8 PhysX system... There is a major issue with the new system
where everything falls through terrains if your scale of your terrain is altered in wed.
In other words you might need to scale your terrain in med so that when you
locate it in wed it has a scale of one... This has caught a lot of us off guard...
Its just a shot in the dark... but... I hope that helps and solves the issue...