Interesting discussion indeed.

Athiril, by saying exposure "thinner" in reference to overexposure, do you mean a thinner range?

As for increasing graininess, I just don't know about that. Larger grains are inherently faster, thus with underexposure you are getting a high percentage of these large grains. It makes sense that overexposure would lead to more of the smaller grains being exposed. Not to mention, this is something you notice in low-light photos, the shadows are often very grainy/noisy. Granted, I've never printed these optically...