I'm not sure I'd agree that you need to increase the tension so much when going to a larger wire size. Lets say the new wire size is twice as strong. For a given load on the mast it will stretch about half as much, consequently the slack side won't have to take up nearly as much either, it equals out. Putting more load on the wire just sends more load into the hull and chain plates. However, it is possible that the hull and deck move so much that it is that motion that the rigging needs to absorb, not cable stretch. In that case you may need to stretch the larger cable by an even greater amount, because the hull and deck are stretching farther too! I'd love to see someone measure before replacing the rigging and then again afterwards.
Gary H. Lucas