While not backing this up by testing, my understanding was, even when the diffraction set in, your image would still be sharper than the less dense sensor's image at the same aperture.
So, and this isn't backed up by math, but just an example, a 24MP sensor at f16 will never resolve less detail than an 18MP sensor also at f16 (assuming the same lens and sensor size). Diffraction, by my understanding is a product of sensor size, not density. That said, if you pixel peep the 24MP sensor, the evidence of the diffraction is more visible, but that is because you have more resolving power.
So, given this assumption, I usually try to shoot micro 4/3 around f4-f5.6 for maximum sharpness. And anything past f11 is noticeably not as sharp. 645 can be shot much higher (f11 or f16, I think). 35mm should be optimal around f8, but I would need to double check that.
I would add, another way to check is to use two bodies with the same sensor size, lens, and settings, then downsize the larger image to match the smaller sensor's pixel dimensions. They should be at least equal in sharpness, with the downsized image being a bit sharper.