I have a fair bit of cold weather experience and have used various down parkas at temps. to colder than -40. I agree that, from -10*F down, a GOOD down parka is best for keeping you warm.

You DO NOT wear lots of layers, three is best, Icebreaker merino  crew, Icebreaker merino Tornado midlayer and your down clothing, with IB merino boxers under Icebreaker merino longjohns. I prefer wool pants to down in most situations, but, the down are warmer for weight.

IF, -15 to -20*F, were the COLDEST I was going to encounter and this is likely what you are going to experience, with 10*F to 0 being your usual "cold" temps.; I would not bother with a down jacket at all. I have used my VERY fine and VERY, VERY costly Richard Egge down double "duvet" from Switzerland at temps. such as these and down to -40, BUT, my Integral Designs Dolomiti hooded parka with Primaloft is a MUCH better choice and my most used winter jacket of which I have quite a number....this IS BC, after all.

I found that my highend serious mountain down jackets/parkas were just too warm to use for much of anything even on long winter camps at cold temps. Try to find an ID dealer and check out some of their gear; it is made in Calgary, AB. where serious cold is a commonplace winter experience, is superb in quality and WORKS!

I now carry Primaloft gear for my emerg. camp, no down, no fleece, no synthetic baselayers, ,just merino and Pl. with an eVent bivy and Silshelter or Silwing. I use the Dolomiti parka from Oct. to April, one of my ID Rundle jackets during summer or my Montbell UL T-wrap and a Wildthings Epic halfbag I just bought. I know from experience that this is good down to colder than I will ever need it in, utterly reliable and very light.