Now that the Sony A7r II has 42 megapixels of madness, it's a good idea to look at how it handles diffraction with a few lenses.
Right now the consensus is that your aperture and megapixel count will have an impact on your image detail due to diffraction. Shooting higher apertures like f16-f22 will result in softer images vs shooting at f4-f8. Also more megapixels will impact how your images look at higher apertures.
However, from what I've been seeing from doing various diffraction tests with different cameras and different lenses, it's not only aperture and megapixel count that impact diffraction.