Posted this on the WoW forums already, but having more input is always good!
Why not make penance be an instant cast ability with 3 charges? Press the spell once to activate, then just hit it again for each charge. Then add a small cooldown between charges so it can't be spammed, while allowing haste to lower that cooldown.
This would easily improve Discs mobile healing capability, and since the spell is already 3 small balls of light anyway you don't even need to change the spell effect!