I understand you. Quantification sure beats marketing hype.
My favorite bags are from WM. Their lightest series/bags are cut a tad tight. I don't mind, but some others might.
I've emailed Montbell this past fall. Wanted a colder bag. Narrowed my choices down to two: WM Versalite Super, or a Montbell SuperStretchDownHugger (#1 or #2). Same day, heard back from Nels. He praised the WM bag i mentioned, and spoke highly of Montbell's. Sadly, only one question was not answered as i hoped it would be. I was hoping for a "number". Instead I got "words". Montbell doesn't list a LOFT value for any of their bags, nor could Nels provide any. I find that Montbell makes fine bags, easy to zip/unzip and mine has yet to foul. Unlike my WM, TNF, and WMtn bags - must keep zipper as straight as possible or 1) it's a real "bear" to zip, or 2) it may catch some fabric and snag. Obviously, this happens only when 1) it's really cold out and you want to zipper up as fast as possible so as not to chill, and 2) when you really gots'ta-go and you can't get out. Maybe i should be gettin' me a quilt? It's tough for me to cp. warmth between similar bags that i own as conditions are different each time out.
The other thing i noticed, even assuming the Montbell 725fp down to be rougly equivalent in loft to the WM 850fp down is that (i can use a WM short bag) the down fill in the WM is divided up along 66", but the the Montbell's down is divided up along 70". Obviously, the difference goes the other way if someone can use a 70" bag from Montbell, but another Mfr's regular length is 72". In that case Montbell's down insulates a shorter length. Just some factors to complicate the matter further. However, having said that, perhaps these factors shouldn't even be considered, and please correct me if you feel i'm mistaken, LOFT is really the deciding factor in warmth, now, isn't it.
Another thing to keep in mind when cp. weight related ratios, is that the Montbell SuperStretch system adds 2oz to a bag's weight versus their non-SuperStretch bags. Fill wt. is the same between them however. So, unless i'm mistaken, when wt. related ratios involving Montbell bags are considered for warmth purposes, the non-SuperStretch bag wts. should be used, or ~2oz of wt substracted from the SuperStretch bags. Obviously, if considering "pack" wt, then the 2oz shouldn't be deducted.