Why take more?
OK...in truth I've never used this bag. I only wish I'd taken it to Denali when I was old enough (young enough) to enjoy it! The "-40" bag I used to climb Denali (one of Earth's coldest environs) was a shadow of this bag. That was 1980 and I didn't know about Western at that time. This bag is astoundingly thick and warm for its 3 lbs. 4 oz. By most accounts, most users of this bag will swear that it's good to -40. Certainly with a tent I would have no question about that. The shell is surprisingly water resistant. Buy the Gore version if you want the ultimate in warmth and water resistance for snow caves and igloos. The inside of the foot section is slightly burlier fabric for placement of boot liners or full boots that have frozen to your feet. (happened to me!) By the way, this is not licence to crawl into your bag with crampons in place! Want the maximum in warmth/weight/compressibility? You're looking at it.