>Downsizing is simply putting both cameras on >equal footing.
Downsizing does more than equalize the sharpness. Downsizing also reduces noise - a lot! This means that if you have a noisy ISO 6400 image from the D800, once you downsize it, the noise disappears. This probably does not require full downsizing to equalize with the size of the other camera, which would then result in a larger clean print, even at ISO 6400.
In fact, this downsizing process is sometimes used to remove grain from film. The negative is scanned at super high resolution, and then the image is downsized. The grain completely disappears leaving a clean sharp image.