I am very new to quilts myself and I actually purchased the new Golite Ultralite 3-season for my very first quilt. I have had it out 6 nights in the Colorado high country and the overnight temps have ranged from 18-25 degrees. The quilt performed beautifully, even at 18 degrees all I had on was a Capilene 1 baselayer, ankle socks and a fleece hat.

I would say that for me, if I added warmer layers such as a Capilene 3 baselayer, wool socks, gloves and my down jacket I could take the quilt down to 10 degrees or so. I am not so sure that just adding a 30 degree bag to this system would add the additional 10 degrees of warmth to get you down to zero degrees-- that is just my opinion.

Personally I would try it out with a bail-out option before you commit to a trip of any substantial time. Backpacking in winter, without the right equipment is not fun.