I tend to think it is not the stiffness of the down itself that is different between 650 and 800, but how much down there is in the garment.

650 occupies less space, per ounce, than 800. So to get, say 2 inches of loft, you will need more 650 down than 800. Hence it is denser than 800. So you could say that the garment is "stiffer"

Garments that are denser will not allow as much moisture through them, so moisture mangagement is more of an issue. Garments that are denser will collect more moisture, so it will take longer to dry.

If an 800 down garment had as much down (weight wise) as a 650 down garment, it would be STIFFER than the 650. But unless you hang out in really strong wind, camp in a wind tunnel with no tent, fall in the snow alot, etc, the extra 800 is not needed.

But extra down can be good for how quick the garment rebounds from stuffed storage. My Helios Vest comes to mind. But I think you don't need nearly as much as a 650.

All insulation works less when compressed; this is not restricted to down. Users need to choose their garments carefully for the situation. Don't expect your lofty down jacket to keep you warm if you lean up against something. We don't expect the bottom of a sleeping bag to keep us warm, that's why we use a pad.

You are right, some garments out there now are not worth their weight for warmth. Products by Patagonia come to mind. Fleece would be a better choice.