Initially I thought nerfing it was excessive since 600 damage isn't all that much, but then I remembered cards like Shining Angel exist and I was forced to think twice...
If not mistaken, Konami made it clear on their in-game announcement that all future cards post-29 Sept maintenance will have their effect dmg halved regardless whether they are OP or not.
Past cards are unaffected except for Lava Golem. (for now)