WM Tamarak Vs. Montbell Thermal Sheet

I purchased this summer a WM Megalite for use as a 2-3 season bag. Now of course as the cold weather sets in i am interested in doing some colder weather backpacking.
Has anyone used the WM Megalite bag or similar and added a 30-50 degree inner bag?
I am looking to go in late November early December in the white mountains in NH.
I would love to just go get a WM Puma at -25 but the cost is steep and its versatility is limited.
The cost of the Tamarak and Thermal Sheet are both $200 or less.
Also would adding a lightweight water proof sleeping bag cover increase temperature rating?
The sleeping bag cover seems like a good idea anyway for that time of year in the white mountains.

WM Megalite 30' – 25oz
WM Tamarak 30' – 15oz
Montbell thermal sheet 50' – 13.4oz
Montbell Sleeping bag cover – 8.3oz

I would appreciate any help or suggestions.

Thanks,

Matt.