Since you are talking about winter and volume - have you considered a pulk? There are a bunch of links on here, this is just one random one:
You need to have a way to bring along your chest full of beer (or maybe hard stuff for internal warming)! But seriously, might be a good solution for bringing along all the extra stuff you might want in the winter.
Edit: oops. I see you mentioned pulks in one of your other posts further down.
In that case for big and cheap +1 on Jam. Use your sleeping pad, which I'm guessing will be substantial, for more support. The bag can handle the weight (within reason) and I'm guessing heavy jacket will tend to mitigate any added shoulder digging.