One thing to consider is that increased tension can cause string slap. Increased tension can result from a changed sight picture which is the necessary result of changing from a shelf cut short of center to a shelf cut past center. Somehow it just doesn't look right to your brain, and your brain compensates with increased tension. Particularly with a recurve, your bow hand should not have a tight grip on the bow: the thumb and forefinger should be gripping the bow just enough to keep it from falling on the ground when you shoot it, with the other fingers trailing off to the side. The same is true of the string fingers: any tendency to torque the string should be evident by a movement of the bow limbs, generally counterclockwise, which should be apparent if there is a very loose grip by the bow hand that allows the movement to happen. Remember, with a shelf cut past center, your arrow shaft is going to be pointed to the right of your old sight picture, and you're going to have to move your bow hand to the left to bring it on target. The bow hand and string hand are not the only places tension could occur, but they are a good place to start looking.