I didn't go with any overstuff in my 20* Pheonix. It has not yet been cold enough in my piece of the forest to find the lower limits. I know I am toasty warm into the mid 40's though. Looking forward to some cool fall nights. I would be interested to know what kind of difference over stuffing really makes. If the standard stuff is enough to fully inflate the baffles, then stuffing more fluff in there will add weight, but not warmth as it cannot trap more air than the baffles would allow.
Am I thinking along the right path here?
"When you see something wobble, push it."