I use a lightweight bivvy and tyvek ground sheet in my floorless shelter. If I know it will be below freezing every night, I bring my 20* quilt, insulated pad, and have a beanie puffy jacket and pants along for the ride. I can easily get into single digits with this set up.
The problem is where I typically backpack hunt, the temperature can vary wildly across a week or two. For this I carry the McKinnis liner bag, a down blanket, insulated pad, pad liner, and my down clothes. I’ve slept in the teens comfortably using everything here - this also included my base layers. The key is the bivvy that helps prevent drafts, and adds a barrier underneath. I’m not sure I could go much colder than this - I had a couple windy nights in the teens and I had a little chill when I woke up.
based on these experiences, If I had to stake my safety and desire to go forward on it - a high quality 20* bag, puffy layers top and bottom, and a liner bag would go with me. If I can’t stay warm with those three layers, I need the hot tent, or I don’t want to be hunting.