"I have a follow-up? Why isn't every bag "overstuffed"? Ie discount the FP to increase down density."
There are too many variables to consider when you try to market this. If you stuck with exact fillpower, you end up with a bag/quilt that is too lean. Here, I would cite the Jacks or Better quilts of a couple years ago. Techically, they were filled with a minimum fill weight allowing maximum "fluffing" or expansion of the feathers. The quilt would develop cold spots over the course of the night, especially for active sleepers. This was fairly well documented a couple years ago. The solution was a couple ounces of overfill. At the current cost of down, this means 10-20 dollars in down. Not something a manufacturor is willing to add without cause. It just costs more to manufacturer.
Adding 2x the fill would about raise the cost of materials by $50-80 in a typical covering. And really have no effect other than to insure good performance if the down is degraded through moisture, dirt, lack of care, etc. It just adds weight, making the bag look like it is a lot heavier than it needs to be. Would you select a 30f bag that weighed 23oz or one that weighed 33oz? By the same token that 2.5x the ammount of down does not really hurt performance, it really doesn't help, other than insurance. It WOULD potentially help if provided more room to loft. It would then be a different bag! I believe the discussion of the article decided 10-15% overstuffing was adequate for most uses.
While down heat retention is mostly due to convectional disruption, larger pockets are less efficient than smaller pockets. This is generally why higher fill downs are better than lower fill downs, there is more branches per plume. However, each branch looses "stiffness". So with higher fill downs, moisture, dirt and body oils degrade it faster than the stiffer lower fill downs. 750FP downs are more consistent over a night than 900fill downs for this reason, in the real world of camping.
Increased density will generally increase heat loss thru conduction. So, increasing the amount of down in a covering might also increase heat loss. I believe this was noted by Mr Nisley and was it was decided that a max of 2.5 was a maximum. Note that sleeping on the bag is a special case. Especially applicable to and around the edges as the down goes from highly compressed to maximum uncompressed, or, rougly 30-40% of a bag. Under you, the feathers still provide fair to good insulation.
One of the worst things that can happen with a bag is cold spots. For example, at 30F sleeping under a 0f bag with your feet hanging out. Your feet get cold and your entire perception is of getting cold. Even though your torso is more than warm enough. This is rather extreme, but just to make a point. This is where "clinginess" or "stickiness" of down plumes comes into play. Eider down only has a loft of 750-800. Yet the perception of the warmth it provides is roughly equivalent to 900-950fp goose down. The difference is the stickiness of the down plumes. The plumes have more barbs on them allowing them to lock together better, forming a more even coating over the entire surface. Expensive as hell, it is considered superior to goose down even at it's lower loft. Cold spots are a big reason that overstuffing works as well as it does, filling in gaps, and edges. 'Corse, a bag will cost between 5000 and 7000 USD.
As far as loft sagging, yes. This happens. Overstuffing does help to alleviate this. But under what conditions do you propose to test it? Air pressure, mosture content, what fabric weight, and so on. No, the only feasable way is to just measure a bag, then let the user decide if it warm enough. The EN ratings usually give a WIDE spread, just don't expect them to keep you warm (forgive me) unless you are a dummy.