I spent a nice night last evening in my Speer Frog Sac. This is a very summer-weight bag/robe/quilt.
I was also using the ZHammock with the overlapped pad system. I used the Frog Sac as a quilt (bottom closed, zipper up to my knees, open in the back). Temperatures were down to 60 with winds 5-10 mph. I felt like the system was good for another ten degrees at least.
I look forward to a 50 degree night in the near future as a laboratory test followed by some real backpacking.