At SL 71, 24 Dex, 10 Str, with leo ring, flynn's ring, hunter's ring and +10 Sharp infused uchigatana; my backstep+R1 counterattacks have done from 620 damage (unbuffed) up to 812 damage (charcoal pine resin) on other players.
Only ~358 AR. And people never expect backsteps in DS3. At that level, most people don't invest enough in Vigor, and the properly timed backstep+R1 easily ends a fight or at the least puts you at a drastic health advantage. I'll probably experiment with the chaos blade to see if I can get up to 900 or more counterattack damage.
Not sure if katana counterattacks are broken or if it has to do with headshots/attack placement, but it's a very underrated move at the moment.