What's with the antagonism? What exaggeration? I just asked what you meant, as it wasn't particularly clear to me. Your original post didn't mention f/5.6 specifically as what "stopping down for assurance" meant.

As for not knowing specifically about the performance vs. a modern plasmat @f/5.6, no, I have not specifically tested that directly. The Xenotar definitely cleans up appreciably one stop down at f/4, specifically the spherical aberrations. I think f/4 or so is the sweet-spot for the lens. Limitations on the DOF would be more of an issue than the sharpness IMO.