"I have always found MB bags optimistic with respect to their warmth rating, despite the restrictions of EN testing."
Also agree. I am a Montbell retailer, (and Valandre) and have tested several of their bags and jackets. While I like their design, I have been disappointed in their warmth ratings as David stated.
I suspect that it could be due to the use of immature down, but that is only a guess.
Valandre', by comparison, uses mature goose down, and I have found their ratings to be spot on.