How far forward did you move the bindings? On my size large I had to move them all the way out to the toe side edge just to center my size 10's. With the burton shrink tech in the Ion's thats a 9.5. That boot must have some serious heel overhang.
my bindings are set at +15/-6 and are centered on the board from edge to edge (approximately). There is minimal heel overhang: