I'm one of those who has used the NeoAir into the 20F temps without an issue - warm and comfy even without the foam pad beneath.

I generally take a wide 1/4 inch evazote when I use the Neo - my general purpose for it is to use in the hammock or on the ground, the wider foam pad will block cold air flowing through the nylon of the hammock while I am actually laying on the Neo itself, partially inflated. But in one instance of a cold and sudden snowstorm blowing up while out car camping with others, I had to pull down the hammock setup and jump in a tent with someone due to the potential danger of dead branches blowing out of trees in high winds - I slept on just the Neo with only the tent bottom beneath it. We woke to snow and I was quite warm all night so it was very much a surprise.

I'd feel comfortable sleeping down to the low 30s without a foam pad on the NeoAir.