[davethebirder]

For the fun of it - I've created a little illustration showing the diffraction limit in megapixels for a given aperture and sensor format (for an ideal lens). The scale is logarithmic.
That's based on 420nm (violet light) as suggested over in Wikipedia. 420nm is a bit optimistic. The human eye's sweet spot is around 555nm (green light). The megapixel limit would be lower in this case - but then you are watching images on a screen.

Keep in mind that "equivalence" remains intact. e.g. the depth of field on MFT at f/5.6 is the same as with f/11 on FF.  And the same applies to the diffraction limit in megapixels (27.47mp MTF @ f/5.6 vs 27.20mp FF @ f/11 in the table below) thus you can't conclude that FF is superior in terms of real-world diffraction.

The data also explains why we usually don't include f/16+ (APS-C/M43: f/11+) in our MTFs charts. Diffraction and not the lens is the dominating factor starting at the settings.

 Given the data it's clear to see that a high mega pixel FF camera is the way to go if maximum sharpness is your aim .... but I'm surprised by the large margin ..........