How thick is this wool blanket? Do you think it would keep you warm down to 20F or lower?

I'd think that a wool blanket (or two) capable of providing warmth in low temperatures would have to be very large and bulky, maybe require more space than an entire large pannier. If I took a wool blanket it would be small, light, and used only to add a bit of warmth inside my sleeping bag (or down quilt) where there are cold spots. I think for a fall-winter trip a down (or synthetic) bag or quilt is the better option.

I have a silk liner and no longer use it. I would not count on it adding much warmth, but it does add more labor when entering a sleeping bag. I think a better option is to take lightweight but warm pajamas. Rather than a wool blanket or silk liner, maybe wool or synthetic fleece pajamas would be a better option for combining with your sleeping bag. When I go cold weather camping, I use a wool long sleeve shirt and synthetic fleece pajama bottoms for extra warmth. If the silk liner is for keeping the bag clean, the pajamas will serve a similar purpose.