I'd rather opt for a light weight warmer bag (in my case the WM Alpinelite, a 20 degree down bag by Western Mountaineering) as the temperatures can fluctuate all over the map year round up there. It could be 70 degrees, walk around the side of a ridge and it drops to 40's...and that's during the day in summer! It's much easier to vent a warm bag than to warm a cold one.

If you are looking for something that probably compresses a little better, might be a little cooler and isn't expensive the Marmot Sawtooth is a 650 fill 20 degree down bag. I have two cold-sleeping friends that have them and love them. They compress almost as well as my WM and likely weigh a good bit less than your synthetic.