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Registeel has higher defenses than Giratina (although less HP). Slaking's attack is as high as Regigigas's. Cushown's constant sleep makes it's behavior unpredictable so you couldn't use it as a wall that chips away at opponents reliably. Also, if it has no recovery moves, then it can't effectively be used as a long-term wall (like Blissey or Giratina). Running some calcs on this mon. There are a lot of mons that could run 3HKOs on it and a few that could 2HKO (Terrakion or Deoxys-A to name a few). In other words, with Sleep Talk randomness and no recovery (not to mention possible SR or spikes) a user would have just enough time to pull of whatever set-up move they were trying to use. Thus, I don't think this would be OP, and I would be very interested in playtesting it.

EDIT: Most +2 Atk or Sp Atk sweepers could 2HKO this thing if they run Close Combat or Focus Blast. So something like that could stop it from setting up effectively.