Have been in Yosemite in September ( around today's date - Sep 11 - it's my birthday) twice in the last 7 years. Night time temps at Little Yosemite Valley were around 60 degrees but at the High Sierra Camps (Glen Aulin, May Lake & Sunrise) temps got down in the upper 20s. We were told you can get a good snow in Toulumne Meadows by the second week of September some years. The Mitylite might be too light for these conditions, especially in October.
I second the idea of a good puffy jacket, especially if you plan on any amount of elevation.
Good luck and don't forget... pictures or it didn't happen. Enjoy!