I have seen an Ultimate Biplane fly with aileron differential, and it did not make the rolls very pretty. They weren't very axial. I can only assume that because one aileron (or sets of ailerons in this case) deflects more, it pushes that wing with greater force. Thus moving the center of rotation. The differential was removed, and the rolls tightened up and looked much better. Also I have never used any aileron to rudder mixing in any of my aerobatic planes, but I don't fly any competitions either. I think they fly fine without it. I have used it in some non aerobatic models though, so I know the advantage of using it.