Well, I'm not sure why he was lying about the class being terrible Neltharia, unless he simply isn't doing well and blames it on the class instead of actually trying to improve. You can clearly see the DPS possible by a good Shadow Priest in the log provided above.
Player issue.
WoL report from a good Shadow Priest if you want to make sure:
http://worldoflogs.com/reports/abau1u0t93c6xmw6/