I can't address the overfill question, not having tried it. I'd be very hesitant to head for the Wind Rivers with a 30* bag, unless you are taking a down parka and pants with you to supplement it.

I take a 20* bag (WM Ultralight). I note from her book that Wandering_Daisy takes a 10* bag. Of course we women tend to sleep colder, but it is quite possible to have nighttime temps in the low 20's or even upper teens even in early August.

I'm looking forward (with envy) to your trip report!