If you did not consistently top the damage meters, you were just not playing your class right. The average shadow priest damage has no bearing here, since the only PVE level Blizzard really need to balance is the top end one, and in the top end PVE metagame, good shadow priests are/were doing a ridiculous amount of damage on any fight with more than one target (for perspective, try naming one fight with one target that isn't a complete pushover).
Only moonkins multidotting rivaled shadow priest DPS, and the moonkins are getting nerfed too.