BD...honestly, if you're talking about the Celsius lineup of a couple years ago, I could agree with you that it was not warm enough for truly cold-weather sits, at least for me. Of course I know guys that stay warm with half the stuff I have to wear.
That said, the current Sitka lineup for the stand hunter is MILES ahead of what it was just 2-3 years ago. I cannot imagine anybody not finding the Fanatic Jacket and Bibs (with proper insulation/core layers) to keep them warm in the coldest whitetail hunting situations. These garments are light years beyond the older Celsius style for frigid conditions.
This year, I didn't feel I could afford the Fanatic jacket, but since I already had the Kelvin insulative layer, I bought the Stratus Jacket. I'm confident that with one or two core layers, the Kelvin mid-layer, and the Stratus outer layer, that I will be set for long sits in zero degree weather. (In my opinion, Kelvin stuff is the best value in the Sitka lineup).
And the Incinerator Jacket (700 wt. down) is a step up from the Fanatic! I've not had a chance to handle that yet.
I would urge you to take a second look at the Forest Line-Up which is designed for the stand hunter. It took them a couple years to iron that angle out, which I suppose is to be expected given that this company originated with the mountain hunters who are on the go, or at least stop and go.
From warmest to least warm outerwear in the Forest Optifade, it runs: Incinerator, Fanatic, Stratus, 90%.
In Open Country Optifade, the jacket steps run: Jetstream, 90%, Ascent. The Timberline and Mountain Pants would fit in between the 90% and Ascent, closer to the 90%.