in september a 30 degree bag with a bivy and a tarp will suffice and be very light. you may need to sleep with a beanie and a vest on but this is hunting not vacation time. I would also go with a blue foam pad with a 3 season 3\4 length pro light inflatable. gives you warmth and some cushion but still light. you can use the blue foam for just sitting around and you would be amazed how much warmer you are with some insulation below your butt.
this was a labor day weekend sheep hunt.