Sorry, I can't give you any great advise but I will say that most people find they are not comfortable in a Highlight bag when using it as a 4 season bag. I don't know if the Summerlight could handle these temperatures in your area.
If you can, I recommend trying the Nunatak Gear bags. I didn't think I could get used to the Arc series but it just comes natural after using it a few times. Try it and you will not regret it.