We haven't seen him in against anyone with the athletic ability and command of distance that Hearns possessed, though...To say nothing of the speed and power.
Possibly. But I woildnt underestimate GGGs athleticism either, as you seem to be doing. What we have seen though - repeatedly actually - was Hearns wobbled and occasionally even stopped by boxers that were no more formidable than GGG. Often less so. For me GGGs strength in depth and greater range of attributes wins out over Hearns more limited - albeit spectacular - ones. In a similar way I see GGG beating Gerald McClellan.
If only more fighters fought like that. Came out swinging ready to kill Duran. Granted he had all the advantages in that fight, but there are others who run from over matched bums, let alone Duran. Duran had no chance. Hearns was too fast and too rangy. Duran could deal with a fast aggressive fighter, like Leonard in the first fight, but the reach combined with the speed left him helpless. Duran's only chance was to go inside from the get go in a kamikaze attack. I have no doubt he had the power to KO Hearns. Still, I don't think he could do it.
I could see GGG getting the KO is he comes in with a Hagleresque mindset. But GGG lacks Hagler's skill in the pocket and on the back foot. He is amazing at cutting off the ring and flurrying against an opponent on the ropes but if Hearns decides he wants to come in and brawl it's very possible GGG gets KOed himself. Also in interest of fairness I have to point out the Hearns fighting GGG would benefit from not having a same day weigh in, he'd be a bit stronger than usual. It's a very interesting stylistic match-up. I think it might be a pretty even fight.
Hearns is better in every away. The problem is, if GGG can take a shot or two from Hearns (huge if), he has more than enough power to KTFO Hearns. If I had to bet, Hearns, easily. GGG's chin really hasn't been tested yet though. Stevens is a big puncher but not enough to land much on GGG.