No, the reason a 900 potency attack from, say, a melee DPS and a tank hit for different amounts is because Strength scaling for tanks is only around 2/3 what it is for melee DPS (or any other DPS and their respective mainstat). This is why a 1,200 potency Confiteor from Paladin will hit for significantly less damage than a 1,200 potency Midare Setsugekka from a Samurai despite both abilities having identical potencies.