I have TNF Tundra, and it is good to -20, if used in conjunction with a VBL liner and adequately insulated. I bought my original Tundra 20 years ago, and when it started to lose it's loft ten years ago, TNF replaced it free of charge, all I had to pay was for the cleaning of the bag. I have been looking at down bags, but I never happen to have $700 burning a hole in my pocket. The weight is about the same, it is just a case of compressability and longevity. Also, the performance when damp is important. No bag is going to perform well if you dump a bottle of water in it, but it is the accumulation of condensation that is a real problem.
I took a North Face Blue Kazoo, with a microfiber shell for a month on the Continental Divide in Montana, and I was constantly fussing with it, breaking it out and hanging it up any time I took a break and the sun was out. I would be fiddling with the down, trying to open up the clumps so they would dry, only to pick up more moisture each night.
I still have that bag 15 years later, and it works as good as new, but I prefer down for shorter trips, where moisture build up is not such an issue.