LBE – minimal balance changes for Heroes V 3.1.

Yes, Dyrman is right, there's no point in weakening the ballista, but adding damage to it would be extremely unbalanced.

That's not quite right. Changing the current "curve" implies weakening the ballista in the early stages, but strengthening it in the later stages.
It's no coincidence that no one has made a mod to strengthen the machines since the game was released, because people understand that it would be absurd.