Loft means loft. 800 fill takes up more space than lesser-volume grades. Don't be fooled by talk about feathers having more resistance to crushing - something some manufacturers used to claim, so you still hear it now and then. It is not true.
Lower count down is lower because it has more feathers. Good down has practically none. Feathers poke through the shell of a bag - or any other garment. That means you have to have a *heavier* so-called down-proof shell. What that really means is *feather proof* or *quill proof*. 800+ down has few quills to poke through so you can use very light weight shells. I typically use 0.8 ox/yd for the inside and 1.1 for the outside of quilts when using 800+ down. Bill Forsnell has used 0.6 or less silk with good results. There will still be a quill or two, but not enough to matter.
FYI: Properly speaking, down consists of plumaceous barbels with no central quill or shaft whatsoever. The barbels radiate excentrically from a central point and the barbels are not straight.
A feather has a central quill (the part that goes in the bird), a shaft (the extension of the quill), two vanes set 180 degrees apart, consisting of parallel barbs held together by regular barbules. The fuzzy stuff on the bottom of the feather is plumaceous barbules. It looks downy, but it is not down. Then there are *pin feathers* which have a quill, some vane and some plumaceous barbules. And finally there are *plumes* which have a quill, a rudimentary shaft, plumaceous barbules but no vane. Some folks like to claim that plumes and pin feathers are down. They are not, but poor quality down will contain them.
The technology for mechanically separating down from feathers, pin feathers and plumes has improved dramatically in recent years, and accounts for the greater availability of high count down. Thirty years ago the separation was done by hand. Incidentally, one such process, air column grading, was invented by our old impractical philosopher friend, Henry David Thoreau, over 150 years ago. But only recently has it been applied to down.