This is my bag, comfort -20, extreme -50C.
http://www.snugpak.com/outdoor/antarctica-re-2511
My poncho liner, which they claim is good for about a 10 deg C gain...
https://www.hillpeoplegear.com/Products/tabid/762/CategoryID/4/ProductID/19/Default.aspx
My Jerven isn't big enough to get into...