I too have a WM Megalite bag.
I've found it OK to +22 F (barely).
But for very cold winter sleeping I'll use either my Mt'n. Hardwear 5th Dimension Polarguard -20 F bag alone or put my Megalite inside at -20 F or lower.
The Megalite is barely a 3 season bag and only then because it's got such great down quality.
Eric
BTW, should anyone here think the Adirondacks of New York are not territory that gets seriously cold let me say that I've experienced -40 F there in 1979 as a Nordic Ski Patroller on the last day of the Pre Olympic Nordic World Cup competition.