John: What locals do in the Canadian sub-arctic is buy two down jackets with offset seams that fit inside each other. You wear both together for -40C to -60C (depending on your metabolism!), or wearing the 'inner' on its own down to -15C or so, the 'outer' on its own for -15C to -30C...
Kluane Mountaineering in Edmonton, Alberta, Canada came up with the inner + outer system.
As well, MEC.ca has very good dual-layer down high loft belay parkas. They also have an excellent Gear Swap site where mountaineering down parkas appear regularly as alpinists have children and retire to the warm rain of Vancouver!
Watch out for down jackets using metal snaps or 'press studs' as these don't work below -20C; also they tend to leave a metal rivet next to the chin or neck---beware, that will lead to contact frostbite!