Kuiu has posted two videos on their blog in the last two days on the durability of the ultra. They ran a frame over in various ways yesterday with a Toyota tundra and it survived. Today they threw it off a hill and all things considered it did well. Mine is on preorder and I'm excited for it to arrive. The whole rockslide vs kuiu thing is stupid and hopefully will go away soon.
I have a fair amount of Kuiu gear, and I love it all. I don't own any of their packs, I have been pretty leery due to the problems they've had with the earlier Icons, but the new Ultra looks pretty good so far. The testing they have shown is impressive.
As far as the original post, I haven't worn a Kelvin, but I do have a Superdown. I am very happy with it. It is far warmer than I expected, weighs almost nothing, and packs down very small. It has been a great addition to my pack.