This subject has come up oft before, and I don't know if I was ever convinced one way or another. And now Risk makes the argument for understuffing maybe = more warmth per uniy weight, if I understand his point.
But 1st of all, please define overstuffing.
800 ci/oz Fp down, if poured into a container, will puff up and fill a container of 800 ci, is that right? And if the length and width or diameter of that container are the right specs, then the hight ( loft ) of the filler area will be, say, 3". So put a lid on the container ( or a nylon cover when baffles make up the container ) that will just barely be touching the down, and now it is niether over stuffed or understuffed ( is that an actual condition? ), but simply "stuffed". It id simply filling the 800 ci of space that it's specs call for, correct?
Now, what happens if we overstuff that container? We put some more down in it. Does the height (loft) of the container remain the same, due to the "lid"? If so, does the warmth supplied by that same 3" of loft now increase? Or does it remain the same or possibly even decrease, due to less trapped air which has been replaced by down? The only thing I know for sure right now is it will be heavier.
Buy what if the lid is movable, not fixed? Or, the nylon "lid" is loose and allows the added down to rise up higher than the container walls(baffles). Is that what is meant by overstuffing? Well, now it is both heavier and has more loft, so I am pretty sure it will be warmer. Though maybe not quite as warm as adding more down AND increasing the baffle height so that the down just rises exactly to the top edge of the baffle? Because the loft will be more uniform and not peaking just in the center of the baffle?
But, if you increase the baffle height(container size ) to accommodate the added down, it is no longer overstuffed, correct? It is simply the new height ( loft ) that the increased amount of down and container size allow. So which is warmer? Enough down to just barely fill a 3" high container being allowed to do just that, or that same amount forced into a 2.5" container? The latter would definitely be more "stuffed" than the former. And denser, one of the reasons, I suppose, of overstuffing. But would it even be as warm, much less warmer?
So I 1st need o know exactly what is meant when someone says overstuff. And if that results in more loft, then I am sure it will mean more warmth. If it does not, then I am not so sure it will. But I don't know!
I can greatly increase warmth in certain areas in my PeaPod, a seriously "understuffed" quilt I suppose, as well as one of the warmest I have used, by shifting the down where I want it. But, there is so much space in those chambers, this does not so much overstuff the areas where I push the down, as it does increase the loft. I can have the rated 2.5" of loft along the 9 ft length, or I can have 4+" of loft is a smaller area.
So let's say we overstuff these walls by a factor of 2 or 4 or 10. At some point, we will have compressed all of the dead air, and the only thing providing any insulation will be the insulation fiber itself, not any dead air. And I suspect it won't be near as much R factor as a lot of trapped dead air.
So, back up some. Let's say we add 20% more insulation than the exact amount needed to fill up the wall space with still uncompressed insulation. We have to stuff it in there just a little bit. We now have more insulation but maybe less air. Will this be warmer? As warm? I have no idea, but I'm sure there is some insulation expert that does know!