Current Darius ultimate gains 68 (+.15 bonus AD) true damage per stack. Yours would give 4% enemy max hp true damage per stack.
For comparison, a 2000 health enemy would take 80 damage per stack. To equal that, Darius would need 80 bonus AD.
A 4000 health enemy would take 160 damage per stack. To equal that, Darius would need 613.33 bonus AD.
So against a champion with NO bonus health Darius would do SLIGHTLY less damage if he built a Hydra and Black Cleaver. With those two items, Darius would do even damage with this change at 2131.25 health. Personally, I don't think massively buffing Darius's damage in almost every case is what he needs.