If economics is the study of macro-psychology, then it would have the same problems as regular psychology: the inability to do truly controlled experiments, only with an even smaller set of entities to build your N out of.
I think the closest we can get is to compare the predictions from their theories to reality X years later. It anyone has done that, I'd be very curious to see it.
What's macro-psychology? Do you mean group-psychology?
Economics is the study of a particular value system within society, that value system being the monetary/capital one. Human psychology comes into play (both at the individual and group level) because human evaluation and action is involved.
Group psychology vs egregore psychology. The difference is whether you are paying attention to individual desires in aggregate, or whether you are attributing desires to the collective entity that no single individual has.
I think the closest we can get is to compare the predictions from their theories to reality X years later. It anyone has done that, I'd be very curious to see it.