Hello Michael, I recently read Ctein's book, generously offered as a free download (linked from Mike Johnson's 'theonlinephotographer'), and thought that I didn't need to do a whole series of tests regarding the focus anomaly of paper, because it would be very time consumming given that I enlarge 4 different formats to 3 different sizes using half a dozen different papers. Hence, the 'simple' test to work out the paper's plane of focus as opposed to the optical one (yes, that's right folks !) would be a major undertaking to have real meaning. However, out of interest, I will try it for one set up to see if there is a significant difference: not sure when, but it is on my 'to do' list.
Only ever under a magnifier have I noticed that a print hasn't been as sharp as hoped and put this down to the point of capture, and here lies the context in which I put the existence of focus anomaly: so far my prints are sharp enough for normal viewing, so I am not alarmed that they could be sharper. However, I am interested in this and so will, as said, explore.
For anyone browsing this, the research by Ctein outlined in 'Post Exposure: Advanced Techniques for the Photographic Printer' showed focus anomalies on variable contrast paper due to the spectral sensitivity of papers receiving a different plane of focus to that of our eyesight. This differs between lens and paper combinations, but is a recognised phenomena by manufacturers. Basically, there is little in lens or paper manufacture to overcome this. However, as said, this is looking at critical sharpness and under normal viewing most prints are probably judged to be 'sharp enough', if they aren't then read the book.

Regards, Mark Walker.