I used a 15 degree sleeping bag for years in the Sierra Nevada and the only time I was too warm was camping at the front country Lodgepole at Sequoia National Park in mid August before hiking the High Sierra Trail connecting to the JMT. I just kept the bag partially open and it was fine.?
I subsequently switched from sleeping bags to quilts. I now have two quilts - a 10 degree from zPacks and a 30 degree from Enlightened Equipment. Everyone is different but for me the "comfort" rating is about 10 degrees more than the actual rating. I have never taken the 30 degree quilt to the high Sierra and doubt I would in the future. The 10 degree has kept me warm and I just open it up if it gets too warm.?
I don't know if I will be able to make a trip work this year but if I do, it will probably toward the later part of September. If I made that trip work, I might actually take both quilts and use the 10F inside the 30F. because I would expect potential nighttime temps in the teens or perhaps even lower where I would be going (Rae Lakes or 60 lakes). It would only be a short trip from Onion Valley for probably 2-3 nights, so food weight would be quite low, and the extra weight of the 30F quilt is only just over a pound.
ravi_jmt2013