I own the WM Ultralight and it is indeed a fine piece of gear, and ditto have heard the same things about Feathered Friends (I have their down booties and like them) and Campmor down bags for folks on a budget.

It might be, however, that a different phrasing of the question would better serve. Temp ratings applied to bags vary a lot, so that one of two 20F rated bags that's lighter weight unfortunately could also be substantially less warm. Typically folks solve this by sort of adjusting their expectations based on the manufacturer quality, so that a Western Mountaineering, Feathered Friends, Nunatak sort of bag is expected to be more "honestly rated"; I think Montbell might be pretty good too, if not quite up to the standards of those others (?).

At some point, the European rating standard may take hold here, EN13537. This article says that in the U.S. only REI, Mountain Hardware, and Marmot have adopted the standard so far.

Another approach is to ignore the rating and look at the total loft (assuming, of course, that THAT is consistently and honestly measured ...). For example, I think that Jacks 'R' Better doesn't give temp ratings for their quilts, but just the loft.

Bottom line though, for me, is that just measuring the weight of the bag isn't sufficient if I don't have a good handle on how warm the bag might really be.

I should also say that if weight is paramount for you, there are some interesting dual use type of approaches out there where you sleeping bag (or quilt) sort of doubles as camp clothing. Jacks 'R' Better as listed above is one example; an Exped Wallcreeper is another, and Nunatak does a different sort of hybrid (half) bag approach.