You'll never get close to matching a true dps, but you can still atonement heal your way through a lot of fights. Queuing as heals and dpsing as disc in Heroics makes them a little more interesting for me. On single target encounters, you can match or even beat the tank in dps and typically keep him topped off.

On Alysrazor, I keep one of my tanks up primarily with HF/smite on the hatchling he's kiting. I shield him when I can and penance on cd. And when the boss lands, you can add an extra touch of dps and top off your raid automatically at the same time (since atonement healing is "smart"). On our last kill, I did 6.3 million in healing and about 3 million dps over the encounter. Nothing to write home about, but it does add up. This is perfect for any boss fight with moments of light incoming damage, damage focused on the tank, or when the raid is stacked. And having archangel ready to pop for moments of large incoming damage is great, though I tend to use it on cd for the mana return. This leaves it active almost half of the time anyway. I also take Train of Thought, since it typically lowers the cd of Penance by 1.5 seconds or so.

The one warning for anyone trying out this playstyle is don't get tunnel vision. HF/smite to your heart's content, but nobody should die while you're at it. Go to your stronger, less efficient heals when you need them.