I've been testing this hypothesis by shooting tethered with a 100% window open so that I can see my focus accurately within a few seconds.
I must say that I have no problem achieving accurate focus using the Schnedier 4X loup on a compendium bellow, 90% of the time. I also defocus and the refocus for a second frame, just to be sure, and thus am getting 100% in-focus frames.
Michael