Thanks for the replies.
I am aware of the EN rating system, and how it is often an overestimate of warmth.
I am actually most interested in the Marmot Plasma 15 degree bag, which has a draft collar ( I believe) and is also substantially lighter than the helium.

I think I want to take the extra weight of the 15 degree bag, even if it dashes my hopes of an under 10 lbs base weight. I'm a big guy, so I can take a litte extra weight, right?!


Edit:

What is the viability of a quilt, bivy combo saving weight and increasing warmth? What do people think about this?