Since OP is looking at a mitten the dexterity/fit advantages of Hestra sort of fly out the window.
I have owned a wide range of high end gloves including the $170 vertical cut freeride Hestras, $150 swany gloves, $140 Leki gloves, so on and so on. So far I have been most impressed with the warmth and the waterproofness of the high end black diamond gloves. I specify "high end" because every company carries price point equipment that isn't representative of their more expensive, all the bells and whistles, line up.
It could very well be that I didn't apply enough balm or bake it properly on my Hestras, but even assuming that they would be waterproof after thorough treatment that is still a lot of hassle to go through. Especially considering my leather black diamond gloves require no such treatment yet still remain completely waterproof, as well as being substantially warmer.
Hestra definitely makes quality gloves, I'm not denying that, their fit system is one that other glove manufacturers should try and emulate. But again, that does not seem to be the OP's primary concern.
Primaloft is synthetic down that was originally developed by the military, who were looking for something that provided the heat reflecting properties of down fill without those properties being compromised when wet. In my experience nothing has the same warmth performance, and almost all of the high black diamond gloves use it. In fact many of the complaints about the expensive BD gloves is that they are too warm.
Seems to me this mitten
would be a good choice for the OP if he decides to try something outside Hestra.