If I have understood correctly Browning tests were conducted against AI fighters.
I can believe that a human gunner can outperform AI gunners against AI planes or against someone that attacks from your dead 6 and parks there... and IMO this is a good thing!
But if the fighter pilot is a human that knows what he is doing I think the results would be quite different.
AI gunners seems to be always able to calculate a firing solution on you no matter where you are or what you do and if you transit for a split second in their firing arc there is a good chance to receive hits.