My bag is a Western Mountaineering Ultralight, which the manufacturer says is a 20*F bag. Its actual EN13537 ratings (found on EU websites, since WM doesn't reveal the info) are comfort, 24*F and lower limit, 17*F. I have taken the bag down to 15*F comfortably by wearing all my insulating clothing inside the bag, over a vapor barrier suit, with an R5 pad. and with a 75 lb. dog curled up against me. That's effectively 10*F lower than the bag's comfort rating. Any colder, and I would have wanted at least one more dog!

A lot depends on whether you're a warm or cold sleeper, whether you can use a vapor barrier and how much insulating clothing you're willing to carry. There can be nights when it gets down to 10*F or colder at higher altitudes that time of year, especially if you're camped down in a low-lying area (remember, cold air sinks).

Remember, those EN13537 ratings are more accurate than those dreamed up by manufacturers' marketing departments!