Hearns was not feather fisted as an amatuer. He didn't punch as murderously as he did at the pro level, but if I recall correctly, Tommy went pro pretty young, and he was tall and skinny. His power really developed when he grew into his frame.

JJ isn't the hardest P4P puncher either. Even in the very modern era, G-Man, RJJ, Tyson hit harder P4P.

Hardest P4P puncher of all time might be Marciano. I know he was an HW, but the guy weighed 184 without cutting weight.