I dont have eric's experience, but from my trails on my back deck, I have to agree. I tried several times to take my 20 degree bag to zero (never really zero but single digits once) with wm clothes or liners, etc. I was never warm. I was OK, even cold, but never warm.
I finally got a zero quilt (and now of course its been 30+ every night) so I havent tried that out.
I seriously considered finding a 30-40 degree quilt to throw over my wm bag, but the sliding kept me from doing it.
Erics snap solution may be the answer.
I finally decided on a 0-10 quilt, and a 25-30 quilt for year round. I figure with my down clothes I can make both of these work in almost any temp. (that I would be out in anyway)