I'm not a raider, but in my experience with discipline in dungeons, I feel your main issue is that you're not using penance as a damage spell. As discipline, as long as the people in your dungeon or raid group have the atonement buff (generated by plea, radiance, and shield), all the damage you deal is converted to healing on the targets buffed with atonement. By not using Penance except as a heal on ONE tank, you're missing out on more people being healed with that Penance. Not only that, Penance is an instant channel, whereas Smite is not. You generate more heals with that first Penance cast