The wiki says:

"The warmth of a down comforter depends not only on the fill power, but also on the quantity of fill."

"A comforter that uses 550 fill power down, for example, would have to use approximately 40%-50% greater weight of down than a similar item that uses 800 fill power down to provide the same loft."

Boy, that muddles up the water when you're trying to decide what to buy.

But, taking that for what it's worth, how does 550 down compare to other types of insulation? Is there anything as light or lighter, and less expensive, that performs as good or better?

And how do your rate feathers that are mixed in? Do they add loft? Or do they add warmth?

They can't do both because then they'd be better than down, and if they add neither, then they only add weight.

I found a down/feather jacket at a thrift store a couple weeks ago. It occurred to me that I might buy it and collect the down and save it. If I could find enough of them I might be able to make my own down quilt for pretty cheap. (the jacket was only $3).

But, honestly, I don't have a clue as to how down compares with other materials. What lightweight insulation besides down do backpackers use to make lightweight gear?