Well, here I go again: 2.5 oz ( I.E. ~ 1 sq.yd of 2.5 oz/sq.yd ) of CS XP has kept me warm into the high 40s, in a WB Yeti sized torso UQ. I think that is spectacular, and I am still a bit unbelieving that that was my result. ( wearing nothing but cotton t-shirt and jeans plus ~ 45F TQ EDIT: and a fleece hat )
Does any one really think that 2.5 oz of 800 FP down in a torso UQ would do any better than that? Plus you would need baffles for more weight. So I really think that, at the least CS is way more competitive with 800+ down than most of us realize. Not in volume/bulk maybe, but in warmth to weight, a lot closer seems to me. And this does not even consider moisture differences.
AHE makes CS UQs. I highly suspect that they are rated way conservative, I should get one some day and find out. I suspect Primaloft is very similar to CS with some pros and cons, Certainly it's CLO rating is very similar, and it laughs at water.
As does CS in my experience. But I know of no one making a Prima loft UQ or TQ. Maybe some one should!