Sleep System

[TwistedCF]

Hi everybody! I'm planning to through hike nobo in 2020 starting as early as Feb 1st with the approach trail. I don't have the budget or the need to equip myself with ultra light gear but I am trying to be sensible about weight. Based on the available choices in my price range I'm looking at a base weight of 15 to 17 lbs but that includes 2 lbs worth of bear canister which I intend to carry the whole way. I'm hoping to top out between 26 and 30 lbs with five days of food.
My question is about sleeping bags. No mater which fill type, the weight seems to go up exponentially as the temp rating drops. I'm wondering if anyone has ever combined a 40 degree down bag with a silk liner and then put the sleeping bag inside an SOL emergency bivy? When combined with a closed cell foam pad would this be a viable system for late winter in the south and then again for early Fall in the Whites? The SOL bivys are more or less disposable but they are priced accordingly and can be tossed and replaced as needed.
The heat index in Ms was 107 this afternoon so I wont be conducting any experiments with sleep systems till Dec. Since our winters are so mild here most retailers don't carry bags below 40 and you see a lot more 55 than 40! If I need a lower rated bag I want to order it asap so I can start packing and use my actual equipment to do my weekly PT hikes this summer and fall.
Thanks...Michael (no trail name yet)