Modifying the grip alone will not increase drawlength or power stroke.

If you add 2" to the grip AND lengthen the draw by 2 inches then your anchor would remain the same. You would add 2" deflex to the riser and he would now have a 7" (false brace).

The bow would be much more touchy and tougher to shoot imo. Its hard to say to waht effect this would have though.