How low can I go?

I use a 15º bag and a 45º bag together, and I can get about 15-20º extra this way, so about -5º. Lower than that, and I need a coat, but with a coat and down booties I can sleep at -15º.

Galveston, huh? You're gonna freeze your Gulf-coast butt off!

Max, thanks for your helpful response. It suggests that I may indeed be able to get by with what I already have, which is always a plus.

Daniel, you may be right; my Gulf Coast skin isn't much inured to the cold. But I've always been drawn to cooler weather backpacking. Last winter, my daughter and I did a December overnight on Guadalupe Peak where the mercury dropped to 16 degrees. And a couple of months ago, I did a week's section hike on the AT in Virginia, with a couple of nights around 20 degrees. While these aren't quite what I'd experience in Montana in January, they're probably a pretty good proof of concept. Plus, who doesn't like tramping through the snow!

A VBL of some kind would be mighty helpful, especially since you will be using two down systems in combo. Without a VBL, condensation would be likely to freeze inside the top bag.

Besides mostly preventing that, a VBL will also add some warmth to the overall system. I don't know how much exactly. I've seen guesstimates here around 10 degrees plus or minus a couple of degrees.

Would recommend a garment VBL of some kind so you can wear your jacket and other clothes on you, and it's good for camp use too. Doesn't have to be expensive. EVA rain gear could work, relatively cheap, fairly light, and fairly durable. A step up in price and durability and a bit lighter would be silnylon rain gear. Polycryo or plastic bags for feet.

That pad combination is inadequate for me on snow. My preference is a full length (62 inch +) insulated inflatable with a half length CCF underneath. One of the Exped synthetic UL 7s or even UL 9s at 22 oz seems like a reasonable choice. Inflate by mouth and dry later at home.

If you are going to go with the Neoair combo I would suggest at least adding another short piece of CFF to double up under the torso. Otherwise your stuff looks OK. No thanks on the vapor barrier for me.

Yeah, I grew up in Houston… When I went to boot camp in Great Lakes in December-February it was the first time I'd seen snow. Then there was that one time in Billings in early March. How the hell do people live up there?!