In reply to andy.tromans:
Never taken my ultras to altitude (so can't help on that front), but stood around in snow etc at -30 for many hours (not including wind chill) and my feet were still very toasty. And the guides are meant to be fractionally warmer, though I do have feet on the warm side anyway.
If boot weight has any correlation to insulation, the Phantom 6000s are only 150g or so heavier than the Guides (though they are a double boot so probably not the best comparison)?
