Coldest I have hunted personally was a couple years ago in Kentucky. There was a really good buck (as far as I know, it was never killed, everyone around was after him). It was in the teens to low 20's and 20+ph wind with gusts. Sat in tree from daylight to dark. Actually used a camo sleeping bag, normal bag, like keep warm to 35 degrees or so, and not many clothes, uninsulated boots, one pair of moisture wicking socks and never got close to cold. Had ordered a heater type body suit that showed up and the next morning it was single digits with a wind but not as bad, sat all day, did not kill a deer but never got uncomfortable either day and I am in a lock on 30 feet or so up, no shooting house or windbreak of any kind. Only difference in sleeping bag and body suit is that body suit is easier to get in and out of, not really any difference in staying warm though.
I have used and still Arctic Shield boot covers for many years, they are awesome. But if it is really cold, or just cool with a strong wind, the heater type body suits are awesome. About the most I ever wear walking in now is: a pair of 5.11 tactical pants, wicking type socks, uninsulated boots, wicking type shirt, camo fleece pullover that zips up the neck when needed, light pair of gloves, hat of some kind (started carrying a dakota dan hat and they are the warmest for head I have ever seen, even when leaving ears uncovered) and either carry coveralls or heater type body suit in and put whichever on once I am in stand. Rarely get even remotely cold. If my feet sweat on way in, I am done till I get them dry and "start over".