+1 to what Stephan says.
Here in the Southeast, I am fine using my original NeoAir on its own down to mid to low 20's (F). (Of course though, we all know that YMMV...everyone is different, and this is what works for me.) Around mid 20's is where I will throw a 1/8" GG ThinLight pad on top of the NeoAir and be fine down into the low teens for sure. However, as Stephan suggests, these are ambient temps rather than ground temps. For areas farther north where the ground is more likely to be colder/frozen, this would probably not be the case.
If I didn't already carry a ccf pad in my pack as my frame support, then I would probably get an All Season to use though, which would eliminate my need to layer a ccf pad on top...
He is also correct about the down compressing beneath you in a sleeping bag. The insulation in the bottom of a sleeping bag provides no additional warmth because it gets compressed the second you lay back on it. Just try it, throw your bag on the frozen ground and then hop in.
However, this is where the Klymit Enertia pad fills in. Since the Enertia pad is a series of tubes rather than a flat sleeping area, the sleeping bag can potentially loft somewhat between the tubes, and provide a bit of insulation. Although I have no experience with these pads, so I dunno...