Regarding sleeping bags: they're a nice to have for a winter kit, but your winter kit should also have insulating clothing (e.g. a synthetic or down jacket, hats, gloves/mittens) that will help a lot and keep you warm in a survival situation even when you're not bundled up for the night.

One add-on option is an ultralight quilt or wearable bag (like the Exped Wallcreeper). I keep one of the latter uncompressed in a garbage bag in my trunk along with the BOB; I can wear it as an extra clothing layer and still move around and use my arms, or I can zip it up as a sleeping bag and still wear additional insulating layers under or over it. It certainly wouldn't be my first choice for a winter camping sleeping bag, but it's great for a BOB.

There are bivy sacks out there that are as light as 5 oz (e.g. from that would make good emergency backups along with an AMK bivy (which works more like a vapor barrier liner and will wet out your insulation pretty fast if you wear those layers inside of the AMK bivy). The ones from BPL or Mountain Laurel Designs (or heavier ones from OR, Integral Designs, etc.) are waterproof/breathable and more suitable as a winter cover for insulating layers.