BD...IMO the early Sitka stuff was not warm enough for cold-weather whitetail (stand) hunting. I think this is largely due to the founders' emphasis on the active, mountain hunting that dominates elk, sheep, and similar hunts...with their intervals of inactivity being shorter in duration and preceded by more heat-producing action. And I do think some people handle cold far better than others (I do not handle it well).
The Celsius line was a step in the right direction, as Sitka began to really attend to the needs of stand hunters too. It met the needs of some, but not for those that get cold easily like me. But give them credit, they listened to hunters who said, "Hey, this is still not enough insulation for stand hunting where one remains motionless for hours."
They solved that issue, IMO, with the Kelvin layers. These kind of have the feel or look of goose-down, but compact much smaller and re-loft with no problems or cold spots, are extremely light, and far warmer IMO than down. I really believe if you've already got some kind of Sitka outer layer (I'm guessing Celsius or 90%), that if you wear some kind of next-to-skin base layer (merino or otherwise) and then put the Kelvin layer in-between, you WILL be toasty in cold weather. The outer fabric is kind of slick, which makes it layer or slide under an outside jacket/pants great. It won't catch or bind, which makes it perfect for that next-to-last layer in extreme cold. And it makes drawing a bow easy.
Kelvin was a 2010 introduction I think, and I've not yet talked to anyone who bought it that didn't LOVE it for what it's intended. (Note, it does have minimum pockets and a slight bit of sheen...because it's not designed as outer wear. That said, my hunting partner absolutely freezes even in 40-50 degree temps. She wears one of my two Kelvin jackets even in mild weather as an outer jacket and never gets busted).
I'd urge you to consider the Kelvin layer of insulation. They are also IMO the most reasonably-priced items on the entire Sitka lineup for what you get. I'd also think they'd be perfect for skiing.