You can gain efficiency by narrowing the last 6" to 1/2" wide and then tapering to whatever width you have at midlimb. Your bow would then be more like a Holmegard. Go slowly and incrementally, 1/16" at a time and exercise the bow and check tiller after each cycle of wood removal.
Or you could tiller it to bend more in the arc of a circle. Either way should gain efficiency which usually means speed and less handshock and a smoother drawing and shooting bow.
Or you could just give to your dad and go shoot
Can I suggest you not chrono that bow with a 615 grain arrow? You'll think less of it. If you must shoot it through a speed trap, find a 5/16" dowel, cut to exact draw length and give it two 3" fletches and a 100 grain tip. You'll love that bow then.