"Of course, that begs the question of why not just use synthetic insulation like Climashield Apex and just use a stronger outside fabric, since it'll be warmer and lighter than wool, also synthetic won't leak if the shell is torn."
I say that with being a fan of natural materials in certain ways, and having experimented with trying different things. For an example, i took an all alpaca throw, folded it over, sewed it up stuffed it with Kapok, and used it as a quilt. It was a small as could possibly be to still cover me, but more so in a fetal position (which i find warmer anyways).
Yeah, it was heavy, but not that heavy, i think it was a little over 3 lbs. The problem was more the bulk.
I didn't actually plan to make this though. I was making a silk quilt stuffed with kapok, but as it was one of my first sewing projects, i really botched the baffles and so just decided last minute before my White Mountain trip to use the alpaca throw. It kept me warmer than my friends, similar weight, old synthetic sleeping bag, but he was a bit obtuse and didn't bring a pad to sleep on for some reason so that might be why he was colder than i was.
Practically speaking, and not eco speaking, An-D's idea would be better weight and bulk wise while giving you a lot more warmth.
If you bring along a lightweight down jacket and pants, and then use say a lighter wool or alpaca or blended throw, you might be able to get away with getting it down into the low 40's depending on your cold tolerance, but i would try it at home in your yard first before going out.