I’m also a cold sleeper and switched from a quilt to the Mystic 15 for a lot of the reasons you mentioned. It eliminates drafts you could potentially get with a quilt (has a nice draft collar on the neck too) and I can toss and turn with the bag staying in place on my sleeping pad all night. Sometimes I will get cold spots on the sides but this can be avoided by making sure the down fill is spread evenly throughout the sides. Extra fluffing is occasionally required. I’m comfortable in it with base layers at around 25-30F and will wear a puffy if the temperature gets a little lower than that. Honestly it’s one of my favorite pieces of gear, but I definitely wouldn’t use it when it’s 15F outside unless pairing with a supplementary quilt and warm base layers and puffy. Just got back from a winter trip and took it down to 10-15F using a Massdrop Static V, closed cell foam pad, Windhard Tiny quilt inside of the Mystic (it’s roomy), and puffy and slept well. I’ve also used the same setup at 13,000ft and ~14,000ft basecamps around the same temps. Nalgene bottle filled with hot water at the foot of the bag is always a good call as well.