So I spent last night in high 30 degree temps with my Doublenest, a thermarest pad, and a $13 Academy sleeping bag rated to 30 degrees. I was in winds that were gusting to 30mph. I was lucky enough to be out by myself and was able to do some experimenting. I also was just wearing a t-shirt, some flannel pants, socks, and a knit cap. It did get cold, but I wasn't really freezing, and got some really good sleep from about 4am-8am. Here's what I noticed though about this bag. The weather proofing, or complete lack of really stood out. A large gust of wind would hit, and you could feel the cold air rush in from the seams and zipper. It's not a mummy bag but did have a drawstring on top which I kept cinched up all night with no condensation issues, which I attribute to the total lack of any kind of seal. I spent the week before in the same setup, near same temps, but less wind, and it was much more toasty.
This might be a stupid question but you almost have to be in the right conditions to tell, are the high end bags better sealed against wind? Is the temp rating merely loft and fill, or does it take any kind of wind breakage into account?