"Now that I am using the thinner sock, my feet FROZE last weekend, and it was only 38*."
I'm a fan of burrowing as well (a luxury of the short), and I also tend to have cold feet. If that is the specific problem you may be better off just getting a pair of down booties at about 2 oz. Also the socks themselves may be the issue if they are too tight as my "thiny-thin" socks tend to be for sleeping. Remember your feet have to actually HEAT the socks and bag in that area, so the better the blood supply the warmer the feet. There can be a bit of a negative feedback thermal "poverty trap" - once your feet get cold, increased vasoconstriction, so colder foot surface temperatures, so insulation is less effective, so colder feet....
Of course smaller total internal bag volume will help, but I'm thinking not so much if your feet are the only issue.
Lastly, again if your feet are the only issue, there is the very cheap solution of a vapor barrier - wrap you feet in plastic bags in a pinch. Between socks is VERY comfortable and warm. Also very useful for putting your feet in the frozen shoes in the morning.