I was looking into this the other day. In the collision checks the system checks at the sides of each 16X16 tile, each tile has 4 points, one on each corner, and it looks like it checks if you cross the plane of a side of the box (if that makes sense). I don't know if this would work but you may be able to remove two of those check and replace them with a single check of the points on opposite corners of the box. I haven't had time to try this yet, just a theory.