I missed that question earlier...
It does make a difference. You of course reach a point of diminishing returns, but it takes a bit more than you'd suspect. Having more densely packed down in a quilt helps it to be less susceptible to radiant loss (like when the wind blows against the outside shell, you can sometimes feel the quilt cool).
Brandon did some informal testing and I think he decided that you could go up to 20% overstuff before it didn't really add much benefit.
Of course most of the UQ designs are pretty good at reaching their given temp ratings, as is. Even adding just an ounce or two will boost it further.
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