For me it's never been about sleeping and trying to keep the tarp warm over night. As others have said, we have down quilts for that.
It's more about having a small protected space to heat my dinner, dry my gloves and socks, poke on a fire while reading a good book before bed time all while it's snowing outside.
Man, just writing that make me want to pack the truck and head for the mtns. I do love snow camping.
If I was making a GrizzBeak for this purpose I think I'd sew the door shut. Use the other end of the tarp to enter/exit.
Less chance of bumping/knocking something over that way.
When I started this project I purchased a piece of 10oz heat treated canvas and used my GrizzBeak as a template it was so darn heavy I needed a tent pole to keep the RL from sagging. I ended up tossing the idea.
Maybe the 7.5oz untreated canvas would be much lighter.