I tried out the new depth sorting feature to understand how it was different. I was hoping that it would make it so UIPanel would automatically detect when a new draw call was necessary, and make the draw calls as needed, even if it meant drawing from the same atlas multiple times due to a different atlas being used in between the sort order.
Sometimes you need to have text in front of and behind sprites from the same atlas, and the font is on a different atlas. That requires three draw calls total. It appears that I still need to add another UIPanel for the foreground stuff and set the z coord a little closer to be in front of the other stuff.