I also am with W_D and Glenn. IMHO, a good night's sleep is well worth a little extra weight. If you're shivering, you won't be sleeping, or at least you'll wake up every half hour. Been there, done that, never again!

AS W_D mentioned, the "average" temp is definitely not a number on which you should base your planning--the lows could easily be 10* above or 10* below that number! (Well, maybe 5*C) I look for the record lows for the dates I'll be there, and pick a number perhaps halfway between that and the average.

As she and Glenn also mentioned, sleeping bag ratings are based on having an adequate pad. For a 20*F (-7*C) sleeping bag rating, the EN13537 laboratory tests use a pad with an R value of 5.