In reply to mountainpenguin:
We were in Levi a few years ago and experienced -30. We found this too cold for cross country skiing. The aerobic nature of XC means you would be breathing hard on -30 degree air. This quickly chills you from the inside out !
Clothing wise, thermals, thin fleece, thick fleece, double quilted down duvet jacket, Dachstein mitts, balaclava and scarf over mouth and nose while walking around outside in -30.
The coldest I XC skied in was minus 20, wearing thermals, Paramo Alta trousers, thick fleece, Paramo Alta jacket, balaclava, scarf over mouth and nose, Dachsteins, two pairs of thick socks. Again got cold from the inside out, requiring stops for hot drinks in the various cafes every half hour or so.
I don't know how warm phantom ultras are, but forty minutes standing around watching the Aurora Borealis in Scarpa Mantas with two pairs of thick socks, had by feet beginning to lose feeling in -30. That might give you some idea.