Along the the similar vein of what Steven McAllister and Bogs and Bergs wrote, it would have to be quite cold for me to be wearing puffy pants. I thought i remember you saying at one point you had crazy cold tolerance in your legs Max?
Even sitting or standing still, a pair of mid weight baselayer bottoms, plus more windproof pants are sufficient for me down to 15 degrees F and i don't have near the cold tolerance i had when i was younger, worked manual labor jobs, lived in MA, and ate lots of meat (i eat vegetarian the huge majority of time now).
Sure it won't be a bit overkill Max? What kind of low temps are you preparing to face? If you have even decent cold tolerance, i guarantee you that a pair of light merino-nylon tights, with mid weight polypro baselayer pants on top of that, and some nylon wind breaker pants on top of those, would keep you down to some pretty cold temps and be a lot more versatile in the sense you wouldn't have to worry about wetting them out with sweat and water vapor like you would with puffy pants. But no, it won't be as light as synthetic puffy pants, especially the higher quality kinds.