Will these be overkill for an ascent on Rainier in May or could I get away with wearing Nepal Cube's. Also looking at the the G5 a little bit more. I know the Baruntse are more geared for 6000M peaks but would rather have the extra warmth incase weather quickly turns south.
Pretty much all of the above boots will work up there during that time of the year. Especially if you are flexible with your plans and aren’t trying to go up regardless of weather. I think the optimal boot for the climbing in this area is the G5 though.