Understanding resolution is not a simple topic. The Rayleigh Criterion in the reference I gave above is the "textbook" example. I got this in a class I took in microscopy decades ago. In this next reference (link at end of this statement) the authors argue that this is not good enough for digital. It is very long but if one scrolls down and looks at the tables (the resolution numbers in the columns go up (apertures decrease in size as one goes down) in each table but the values vary according to criterion - going across in the table (for a given aperture)) it is obvious that the maximum theoretical resolution (i.e., diffraction limited) at f/2.8 is greater than f/5.6
Theoretically that is obvious, from a practical standpoint aberrations and mp limits cut in way before that when wide open for us camera users and the lenses we actually have available. Your assumption earlier was "assuming similar correction for lens aberrations", it is more than four times more difficult, many would say sixteen times, to manufacture a 400 f2.8 than a 400 f5.6 with the same optical aberrations.
Theory ends when "limited" purchasing options are all we have.