I think it depends on an individuals cold tolerance a lot. This last early spring, I cowboy camped at SNP with a low of 37 with just a one side tyvek and one side bug netting bivy, a light/thin base layer, a mid weight one on top, a pair of linen pants with light weight silk bottoms, a Stoic hadron down anorak jacket, and a lightweight merino balaclava. Slept on a BA ICP. NO quilt or bag. Yeah I was a bit cold, but I ended up hiking back to the car in the middle of the night..not so much because of the cold, but because the bivy I had made was pretty new and I hadn't properly treated the home wrap yet and so it was quite loud and I felt like I might have been disturbing some thru hikers who weren't that far away, so I left. Realistically I could have used a little more wind protection and insulation. But I could easily take Stephens system down to freezing provided I add in the merino balaclava, little wind or had some basic wind protection like a tarp. I do tend to run warm though. Some might be quite cold in that same set up with those conditions. I know my wife would freeze.
