In reply to Arbu:
I'd agree with most above that a light-medium sleeping bag + down jacket is better for what you want. Down salopettes are rarely used anywhere and would be useless to you on the rest of your trip, whereas just one suitable sleeping bag could be used on all nights, with the addition of the down jacket at your high camp.
I've slept in just a down suit at C1 on Gasherbrum 1 and also on the summit of Vinson in Antarctica. The latter was measured at -46C. It was a right shiverfest and we didn't really sleep much over the 6-8hrs. Not recommended. I did it on another high peak in Antarctica, in a down jacket and Primaloft pants, but wedged between two others who were in big bags. It was OK, but still not great. It can be a suitable tactic for some climbs in certain areas, but I don't think your situation is one.
Your Rab Expedition parka probably is overkill, with the caveat that Klyuch has a monster summit day - around 1400m? - and a prolonged descent or exhausted bivi might be nicer with a big jacket. Watch those lava bombs, someone was killed 10 years or so ago struck by a flying lump.