Stephen,
At the time of my Arapaho Pass trip 3 years ago my Megalite did not have overfill. I had it overfilled 1 oz. (850 cu. in.) last fall so I will likely get at least a 5 F. lower temp range. BUT… with the overfill it's now not possible to shake down from the bottom to the top and vice-versa. The bag now looks more like the Michelien Man.
As I said earlier, I wore a Thermolite insulated jacket and pants that night. Because it was a Megalite bag (wider torso) there was room for that insulated suit. I think with the overfill it is actually good for a 10 F. lower temperature rang. i.e. 20 F. but I haven't had the temps to test that assumption.

