Two results can happen. It is possible that no peak savings will result. Alternatively, the occupants tolerate the warmer temperatures during peak periods, allowing the air conditioner to begin cycling. That cycling translates into some peak savings. Duct repairs can effect interior humidity levels.
Air conditioners have different dehumidification capabilities.
However, brief operating cycles generally remove less water than long cycles. A properly sized unit will probably be a more effective dehumidifier than an oversized unit. It is difficult to predict how humidity will change after duct retrofits, because it will depend on the system's characteristics, the location of the duct leaks, and the climate. But don't be surprised if humidity does change. Unfortunately it is difficult to determine if the air conditioner is properly sized without visiting the house during the peak periods.