geokite
I do not know "how much" effect there is. BUT
When I was a kid and hiked over 400 miles in the Sierras one summer long ago, I had a US mountain regular down bag. It was stuffed with "waterfowl feathers" and 50% "Chicken feathers". It was warm, didn't compress too well, but always maintained its loft because the stiffer chicken feathers held the cotton shell in place. There definitely is a cost.

RE FLAPPING
Someone said they didn't notice their 800 bag flapping in wind. Ok, even if it doesn't "flap" it will be deformed by wind pressure and be thinner on the windward side. A shelter is a neccesity if using very "soft" sleeping bags. The point is that a "stiffer" fill means less deformation and requires more wind to flap or deform. In sleeping bags in shelters it doesn't matter, same as 800 jackets worn in the shelter, but in the real world 800 down is TOO compressable - its is over engineered to the point of diminishing returns. <img src="/forums/images/graemlins/shocked.gif" alt="" />

If manufacturers had used the same weight of 800 down as 650 down (more insulation same weight), the "pressure" inside the down would be adequate to maintain the garments virtual shape, BUT manufacturers are following consumer pressure to create " UL - less wind worthy" garments at any cost so they hit some magic weight number and they can make their sales against the other company that produces what people need. What I am saying is that this desire for UL down jackets makes no sense. A down jacket is a last resort, a safe haven, why skimp on it? <img src="/forums/images/graemlins/crazy.gif" alt="" />

My wifes big down coat is from REI, 20 years old and probably stuffed with 550 or 650 down. It is always puffy and warm. <img src="/forums/images/graemlins/grin.gif" alt="" />

As someone said - 650 down is for stadium jackets. <img src="/forums/images/graemlins/smile.gif" alt="" />I consider my camping to be more "stadium" oriented [outside the tent] than "hide in the tent and sleeping bag" oriented. I like to play in the snow, crawl through snow drifts <img src="/forums/images/graemlins/grin.gif" alt="" />and stuff - try that in your UL winter gear.

Jim <img src="/forums/images/graemlins/cool.gif" alt="" />
Jim