You're definitely capable of doing more DPS, however not all tank classes are equal in all regards. Comparing yourself to say a Druid or Monk may just be silly. You should really be looking at yourself on say World of Logs and comparing yourself to other Paladins. Don't fall into the trap of comparing things that are inherently different. As an example, solo tanking Sha of Pride I (a 570 Druid) did nearly double the damage of our other tank (a 569 Warrior) who solo tanked it the week before (they did 52% the DPS I did). And before you ask, solo tanking this fight is possible once you can force the 30% transition before the second Swelling Pride. You should really be comparing yourself to other Paladins.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.