Good point re: focus shift -- I did the test outdoors and had to stop down partially. I'll do it again in lower light tomorrow.
I just looked through the viewfinder a few minutes ago, and when I focus and then press the depth of field button, the split prism (which works down to about f5.6) is way off. I say off, because it doesn't agree with the ground glass; and at long distances it would call for focus adjustments that are plainly absurd. I guess the plot thickens.