Tom, the one inherent factor that's affected by raising the height of the sub is the degree to which the floor/ceiling room mode is excited. Raising it to the midway point reaches the minimum for that height mode, just as the midpoint of the side wall is the minimum for the length mode and the midpoint of the front or back wall is the minimum for the width mode. Of course, raising a sub about 4' involves more work and possible aesthetic concern than the mid-wall placements do, and might not be justified by the mode benefit.
