>Actually I have to say that as a rule of thumb down retains its loft much better than synthetics. All my expensive synthetics have lost loft to noticeable degree, while none of my down bags have. And they have been used, a lot.
I agree with you wholeheartedly,Bruce, but notice what the original question was. Colin wanted to know about the minimum-volume bag/ compression bag combo. Compression bags/ sacks are deadly to down bags (and synth bags, as well), but they will crush the life out of down. In my misspent youth, when I was carrying 65 pounds, I killed both down pillows and down sleeping bags that way.
You put your bag in a stuff sack, not a compression sack, and you leave it in the stuff sack the minimum amount of time that the actual hike forces you to. That's the right thing to do to protect the loft of any bag, but it will lengthen the life of down considerably.