Some body (the Jacks? not sure ) has previously provided their definition of overstuff as being the amount of down to raise the "loose" fabric that spans the distance from one baffle to the next X % above the baffle.
So, normal stuff would be the minimum amount of down needed to fill up the space in a chamber with, for ex., 3" baffles. The down would rise to exactly 3", theoretically any way. So how would that work? 1 oz of 800FP down would fill a 16.34" square chamber to a height of 3", or that same chamber with 3" baffles filled right to the rim? Is that about right?
If there is a somewhat loose piece of fabric, capable of expansion, stretched over that 3" baffle, and enough down is added to rise the "loose" nylon shell up above the baffles by another .6", then that is 20% overfill. Is that right? Anyway, I think that is the definition at least one manufacturer uses.
With that definition, 20% overfill gives you the approximate warmth of 20% more loft. Not quite, because the chamber above the baffles will have max loft in the center and less right above the baffles.
If the chamber is a square above the baffles, with no room for expansion, then what does adding more down beyond the amount needed to fill the chamber do? I'm not sure it would be warmer, as adding more down would replace the air which does the insulating, though it might not decrease warmth until some point. But maybe there are other benefits that would increase warmth? Wind resistance?
Did I also read somewhere that all JRB quilts have at least 20% overfill? If so, Hudson River or Sierra Sniveller 2" baffles + 20%= 2.4" loft(actual rating 2.5). But, MW4 3" baffles with 4" loft , more like 33%. And the MW3 has 2" baffles and 3" loft, so way more "overstuffing there. 50%?
But for one thing, we need to make sure we are all working with the same definition of "overfill". Are we talking about adding more down into a chamber that can not be expanded? Or are we talking about actually increasing loft above the baffles?
If we mean increasing loft above the 2" baffles by 1" or whatever example, then performance should be increase nearly the same as using a quilt not filled beyond it's 3" baffles. So, maybe 15 to 20*F warmer.