Its definitely that op, but you could if you managed to get Diavolo into a scenario where he was in danger of dying for up to 10 seconds, like in the middle of a field with 11 seconds of constant bullet firing from many many angles, or a massive amount of napalm. He can see 10 seconds into the future, so at time 0, he sees the bullets firing, and Diavolo starts to stop it, when it reaches time 10, KC begins to skip time, but he cannot go past 10 seconds due to a cooldown he inevitably has.

The problem in the matter is taht Diavolo is inherently paranoid and it would be nigh impossible to get him into such a position.

He said it himself, that if the enemy knew of him and if there were a good number of them, then he would lose. The ability just seems more overpowered because his fights are either 1 on 1, or when his opponents are unaware of his presence.