I own one down jacket (I talk about that in your other down jacket thread) and I don't backpack or hike with it. Not because of compression but because layering is all but impossible with down jackets. If we're just talking about standing around in the cold, I find down and heavy wool to be warmest for me. Things like watching a cold soccer game with my kid's, or fishing,...inactive stuff. Leather jackets are worthless.... <img src="/forums/images/graemlins/smirk.gif" alt="" />
For hiking, I need the ability to shed or add layers.

Caveat about down...and the way I use down while hiking....many times after breaking camp, usually in the dark, and the morning chill is still in my bones, I've wrapped myself in my down quilt or bag and hiked on, or enjoyed breakfast. As long as there's no bushwhacking or snags, that quilt or bag covers me head to toe and is crazy warm until I "warm up". Then it gets stuffed in the pack. A down 'robe' is a good description. I suspect we've all done this at one time or another.

None of my 5 down bags are over 700 fill power and my favorite bag was bought in 1984 and is 650fp. It's still my favorite, has been machine washed and dried countless times, and is still as fluffy as new. It's an old TNF Chrysalis that's now a quilt (zipper mods). I've often wondered if high fill powers like 900 don't eventually settle down to the 6-700's after a few years of closet life. With any piece of backpacking gear and it's UL factor, there's a line between utilitarian and UL. Heavier fabric does crush higher fill powers. Lighter fabric won't hold up.
If we're wanting the lightest of everything, Aerogel and mylar would be the way to go, but you only get to use it one day. <img src="/forums/images/graemlins/grin.gif" alt="" />

All that said, I've never been colder than -5 F in my whole life and hike mostly desert, which has wild swings in temperature. I can't speak for you mountain/snow people. Down may be THE thing there. I've been caught out in snow...happily and gratefully....but never hiked it on purpose.