Hagler has a legendary chin, and Hearns was at middleweight which is not where he was at his most powerful. There were times during their war where Hagler looked at least respectful to Hearns' power (and yes, he did break his hand early in the fight). Hearns' power at Welterweight ranks up there with the best (if not THE best) all time at that weight. Let's also take into account, sure SRL wasn't knocked out but Hearns nearly ended his career.
Hagler and Leonard have ATG Chins, so no, the fact that he couldn't KO either doesn't reflect on his punching power whatsoever. Hearns hit like a ****ing freight train.
I do not see why that is a good question, look what tommy did to Duran. Also, the guy carried his power up to cruiserweight, which is un-****ing-believable