Barry
As you point out, the WM products seem to stand up. This IS construction technique AND a quality company that probably simply puts more of the down in their products.

In my own humble opinion, 900 down that lofts at its max to say 100 cubic inches and 650 down that lofts to the same 100 cubic inches has very different material in it. That 900 loft is probably optimistic and is like testing stoves in the warmth of your kitchen vs in a cold wind. Where 900 may be "better", what does better really mean? Maybe you have to treat 900 down as though it were 800. If you only use a tiny amount of down because you "Believe" what you hear, then you may be fooling yourself.

As I have stated I would rather have a 700 down sleeping bag than a 900 down sleeping bag because the 900 is going to collapse under any weight of pressure BECAUSE IT IS SO CAOLAPSEABLE/EXPANDABLE. Its like fleece for example, it doesn't compress much, therefore it also doesn't lose its loft. I think the extreme desire for hyperlight items is clouding the reality as reported by observation.

Just my $.03 worth
Jim