From a healer perspective, yes it would be OP.

To be fair, I can understand where you're coming from. You've got a skill that you make sacrifices to use and its only semi reliable. But look at it from the healers point of view. We dps because there usually isn't enough to heal to begin with. We have limited accessibility to saves in the event that we, or the person we're healing, makes a big mistake. If clemency was instant, it could function as another save, substantially lowering the skill requirements for healers. Being instant also guarantees the cast and encourages the use of the skill, meaning that in less difficult content, there would be less stance dancing and less healing for healers to perform.

It would be OP, because it steps too far into the healer role.