I currently have two decent 25-30 degree bags that I like. However, in the winter I may need something to keep me warm down to 15 degrees.

I'm thinking of purchasing a 1 pound bag like the Marmot pounder to use as a quilt in conjunction with my 30 degree Sierra Laser bag.

Is this a good idea? Some of these bag "inserts" weigh almost a pound anyway, but I just don't know if putting a 40 degree bag over me will surpass what an insert will do.

I use a 30* montbell down bag with a 35* rei down bag modified as a quilt (zipper and hood removed) down to the twenties. I'll qualify this by saying I sleep cold and can only use the 30* bag down to about 35*-40*.

The montbell is 23 ounces and the rei is 15 ounces. I use the quilt in the summer and the montbell in fall and spring (Tennessee) and both in winter. I was able to get the rei bag on clearance for $80 so that was a better financial solution for me than to get a new 15-20* down bag to use for just one or two winter trips each year.