EN rating for my Marmot 0 degree Never Winter is right on. It says 17F comfort rating for women, and hey, I got cold right about that temperature.

You'll want something good to 20F for the Sierra in summer, and the clothing you are taking should in addition to the bag take you lower than that. I've seen the weather turn - one trip the forecast when we started was a full 20 degrees warmer than what we got.

I have a quilt and never use it with a bivy. It's a Jacks R Better Hudson River. It's warm to about 25F quite reliably (I had it out with me a couple weeks ago when the water turned crunchy and we woke to a hard frost, the low had to be a sustained mid20F to get the water that way, and I was warm all night). However, you have to be sure your quilt can fit all the way around you so you can tuck in around the edges. I always take care to use it with an adequately warm pad - in my case either the NeoAir (original, not the current all season ones) or a Big Agnes Q Core rated to 15F. And you need a hat. But I'm generally warm in a base layer with just the quilt and a wool beanie.