Hi all, just a couple of observations:
A medium (6 ft) nunatak alpinist (sleeping bag, not quilt) has 11.5 ounces of high quality down fill and is billed as a 20 degree F bag.
A medium WM summerlite has 10 ounces and is rated by Western Mountaineering to be a 32 degree bag (i use this bag).
The WM Ultralite has 16 ounces of down fill and is rated to 20 degrees F.
All of these bags have roughly similar girth/length dimensions, with the WM bags actually being slightly trimmer. This leads me to believe that "20 degree bag" means different things to WM and nunatak. In my estimation, if the alpinist were rated using the same criteria as the WM bags it may be a 28 or 30 degree bag, based upon fill quantities.
Also, as was alluded to above, you'd have to add a heck of a lot of overfill to a Summerlite to end up with as much down as a WM Ultralite (6 ounces!).