I've been just overly impressed with how darn warm the Kelvin layers are. A little "history" (what I teach, by the way) about Sitka is that they were begun by guys who make their meat mostly on spot and stalk hunts. They climb mountains and kill big elk and muleys. Stuff I don't yet get to do being a flatlander and teacher in NE. When they began to appeal to bowhunters of all species, I did feel they were deficient in gear for the tree-stand man, the guy who is immobile for 2-4 hours or more. Then they came out with the Celsius line, which was a big improvement and a step in the right direction. And now they have Kelvin and other products that are perfect for the stand hunter. I've filled both my NE buck tags this season, but am anxious for Christmas break and a chance to see how the new lines perform in very cold weather. What little cold weather we got before December, I was totally impressed. And when I wear the Kelvin coat for around-town wear (it is comfy!), there are times when I'm roasting inside it...so I know it's gonna be great for the ultra cold weather. It's already performed great in the 20 degree stuff originally asked about, and I have no doubts with the proper layering done, it will perform in the really cold stuff too. And like Steve or Mike or someone mentioned, the good thing is if you buy the right size, you are not going to have issues with binding. The layering "system" works.
The other comment I might add is that I really do think this Sitka Gear will outlast by many times the other items I have bought over the last 30 years of serious bowhunting. I generally hunt 75-100 times for deer each season, and throw in the other stuff/scouting/etc. and my gear gets used. So far, totally happy.