You've probably received enough good info here already to decide what you need on Baker in August/early September: good pad insulation & bag size is whatever makes you comfortable allowing for adding clothes to make warmer and zipping open to exhaust excess heat... I'd probably err to the warm side with a 15-25df bag.
However, on an extended trip, where you don't stay in one camp, but keep moving (tours, traverses. etc.), your down bag will become damp and lose loft due to your own perspiration unless you use an inner VB liner. But, that, perhaps, is for another discussion.
Using down-filled articles and not being obsessed with weight to the point of cutting off toothbrush handles, I've come to enjoy using these bags for down articles to decrease pack size (which allow for a net weight reduction).