So having played Winston and now understanding his playstyle, I can honestly say...
...he still needs a buff. Just to get him more in line with other tanks' damage output. Not even a large one, just a slight buff to his gun.
His time to kill is depressingly slow. The perks of passing through shields and harming multiple enemies are good, and flanking and jumping around is so fun, but it's hard to really use your gun effectively since things take so long to kill with it. Feels like it should be more of a shield counter with its pass through effect, but you are not killing tanks with that thing, it's too weak.
It's depressing when you're spraying down low health targets and it's taking so many seconds to kill even the weakest enemies.
Reinhardt's Hammer and Pin, Zarya's fully charged cannon, Roadhog's mean shotgun, even D.va's guns outclass Winston. There just feels like so many more better options for an offensive tank honestly. And that's while loving Winston's playstyle.
No offense, but you are just wrong on this. Not only is 1 winston great by himself, 2 winston is completely overpowered. Two winstons doing 100 aoe dps to the enemy back line is absurd. I feel like you are discounting the fact that the gun does aoe dps. So of course it will feel weak when trying to kill single heroes... But even then, leap does 50 damage, and your gun does 50 dps, melee 30 dmg. That's a ~2.7s kill time on 200 health heroes with great consistency since you don't have to aim well to hit your target.