I have one of these sleeping bag systems left over from my recently-ended time in the military, and if you read the labeling, it shows the temperature ratings for each piece separately and together, with no mention of how the bivy works into it. I don't remember the exact numbers, but I know from experience that I used just the green piece as a TQ (not zipped up, just laid over me) and a military issue sleeping mat in temperatures in the 40's-50's and I was very comfortable. I was also wearing polypro long johns, socks, and a knit cap, too. I hope this helps.
- We're never going to survive this!!
- Nonsense. You only say that because no one ever has.