> there would need to be a several studies where a large diverse groups of "norma"l health subjects were assessed using some standardize cognition test(s) and then given some stimulant medication and reassessed.

Been done, dude. Also been done in rats (with respect to the impact of stimulants on learning / memory for maze running).

You seem to be assuming that there is some inner dysfunction that will show up in the brain. Some categorical difference that marks whether people have ADD or not. Maybe ADD is more like... Blood pressure. Which is to say that there is a continuum... It might be that the population exhibits something of a bell curve with respect to both outer manifestation (ability to focus, blood pressure) and inner causes (e.g., number of dopamine receptors and / or amount of dopamine etc). If that is the way it is (and that is the way it seems to be) then there simply won't be a magical dividing line between people who have and people who don't have. Where do we choose to draw the line? How high do we want prevalence to be? When do we decide to say that a difference is a dysfunction? Is that a decision or a discovery?

Difficult questions...