Jim's suggestions on clothing will cut your weight some but may not be right for every condition. Here in Minnesota winter camping can mean pulling pulk over either river ice or lake ice. In those conditions, where the possibility exists of getting very wet, I wouldn't leave layering systems at home. They may be your salvation if the ice cracks and you get wet.

Similarly, a bag and overbag systems can work quite well. I believe that Phat uses such a system. Why do you need a bivy bag in addition to the bag and overbag. Can that be left at home?