>Yes, the optimum shutter speed (in your terms) will vary >depending on the degree of crop - because with a deeper crop, >any given amount of subject movement will form a larger >proportion of the remaining frame width.
Exactly. If you are finding cropped shots to be crisper at higher shutter speeds, it is still motion blur that is the issue, not pixel density. (Of course, that motion blur may come from movement of the subject or from camera shake.)
Look at it this way: If pixel density is insufficient, the whole image will be unsharp, not just the parts in motion.