I have the WM Antelope (+5F rated) and wouldn't hesitate to use it at 0F with a warm pad. I get too hot in it with an R-4 pad around 25F without heavy base layers. Haven't had it down colder than that yet, but it's got more loft than the -30 bag I (unpleasantly) used at -40 in Alaska years ago.
Assuming Kuiu's claimed specs are accurate (I know from experience that WM's are), I think their bag is closer to the Antelope based on fill weight.
The fact that Kuiu is using vertical baffles in a sleeping bag tells me that they don't understand what they're doing. 95% of the feathers you're carrying in the bottom baffles are just wasted weight, and might as well be subtracted from the warmth of the bag. With all my down bags (all of mine have horizontal baffles, including my $80 Kelty Cosmic), I shake all the feathers in to the top before I lay it on my pad. I stay a lot warmer at night when I do this (in the summer, I do the opposite to avoid getting too hot at night). Kuiu's bullet points such as "• Maximized warmth to weight ratio" further tell me that they're spewing nonsense marketing jargon. Considering you can buy a standard Antelope for $200 less than that Kuiu bag, or a gore-tex Antelope (that's what I have) for $100 less, I'm led to believe that Kuiu is smoking crack. After watching their video trying to invent a need for hydrophobic down, I'm not surprised. Anyone who has ever tried to hand wash a down sleeping bag will realize that getting feathers wet is not as simple as it sounds...and that's not to mention how practical that would even be when it's 0F and so dry out that your nose bleeds.
