Phil gibson,and associates , at Natick Labs published work on the thermal conductivity of down and some synthetic insulations as a function of compression.www.jeffjournal.org/papers/Volume2/Gibson.pdf
As the down is compressed to higher densities, the thermal conductivity declines (thermal resistance increases) which can be seen from the figures.
This paper does a poor job of describing the materials used. It appears as if the down used was 550 fill power based on the lowest down density listed in the figures. No, the authors never specified the down quality.
Some "down experts" have argued that when you design a garment with 800 Cu.In./Oz down, commercial experience is to overfill the down chambers slightly to give them a little "Puff" feel.
Any overfill makes them slightly less (very slightly) compressible but the loss of thickness from compression is made up partly by lower thermal conductivity of the compressed down.
So far, almost all of the papers I have read on down are full of vague spots, such as not telling you what down is being studied as in the above example. Sometimes, as in the above paper, you will find descriptive claims that are marginally correct. You cn find "down experts" who leave you more confused than you were before you talked with them.