I do use a separate down jacket, simply for the convenience of having a jacket at camp. I am fully conversant with the efficiency angle and it might be costing me maybe 8oz or half a pound. But handling a quilt that is not designed for walking around in is a painful experience. Typically, I carry a 20F quilt in a long/wide(about 21oz.) And a down jacket at 12oz. If it gets down to 20F, I use my hiking cloths as well as my long underwear, and, my jacket. I know I will be good to 30F with no help from other clothing (hiking cloths) but still use my jacket as supplemental warmth. I am getting older so I have to pee 3 times every night, so it is nice having the jacket already on to head out into the woods.
Anyway, loft for 850FP and above down is not the only determining factor of warmth. 950FP has finer, more flexible fibers and more of them on an average down plume than 800FP down. Loss of loft would be less critical for the 950FP down, because, it breaks up trapped air more efficiently than 800FP down, reducing convective losses inside the quilt…more or less. Convection/conduction is the primary thing you are trying to reduce. Smaller air pockets are more efficient at reducing convectional losses. 800FP down also has a higher percentage of rachi which conducts heat better than plume(filament)/air does. The rachis are found in semi-down feathers not true powder down…of course, 800FP down has more semi-down than true down. 800FP may loft a bit higher after some hard use, but the air pockets are also larger, rather than 950FP down which may show some degradation in loft, but, the smaller air pockets will be warmer than the larger ones in 800FP down.
Hey, it is a bit geeky, but 950 will last longer under the same conditions, even though it will loft something less than 800FP. Of course, there is always the dividing line between convectional loss being overcome by simple numerical quantity.