I went back and checked my notes from conversations I've had with Western Mountaineering and this is what they have told me about the down overfill option: There will be less down shifting. There is a small temperature rating improvement. There is a small loft improvement. There is a small improvement to long term loft (bag life). If you are a side sleeper that moves around a lot there will be more down on the bottom of the bag that will end up on your side during the night.
With all of this said, WM does not believe it is necessary and all of the changes are minimal. Less down shifting will be the most noticeable difference.
With all of this said, WM does not believe it is necessary and all of the changes are minimal. Less down shifting will be the most noticeable difference.