No way could I get below 60 without something added in the Clark NA unless it was totally windless. I've been out in as low as 25* (overnight low) and that REQUIRED a good, fluffy, underquilt and a 20* sleeping bag. At those temps the pockets having anything in them works a bit against you by creating voids in the under insulation and making cold spots all around the edges of the pockets.
My legs would freeze into cramped-up ice if I tried to get into the low 50's with just the pockets insulated. Now I do think the Clark NA has features that make it more winter friendly. The weathersheild is a big one. Huge difference in interior warmth and also not having cold air sinking right into your face. You'll notice the difference right quick when you unzip and step out. The pockets do add warmth because it gets hot on my back even on cool afternoons and you can feel the difference in a breeze between the legs and the torso.
My experience is:
75 and up - Nothing really needed but a top sheet.
65 to 70 - A fleece blanket under me and a sheet.
55 to 65 - Summer underquilt and summer sleeping bag, fleece blanket.
40 to 55 - Heavy underquilt and summer sleeping bag or top quilt. weathershield.
25 to 40 - Winter underquilt and sleeping bag. weathershield.
I don't think I could keep my feet warm after 25 and just barely tolerable for me at that temp. That is my breaking point. If my feet get cold and the cramping starts then there is no sleeping. For dead of winter camping... like Doe season here in MO I will usually still take a tent. Easier to keep the feet warm at night. I won't go backpacking in winter only car camping at the most.