Simulationcraft calculates dps on patchwerk fights without interrupts and then the difference is max 30k dps (compared to arcane mages). You also need both players to be of equal skill and have equal gear. How often does this happen? I have yet to see a patchwerk fight in MoP. There is always movement or adds and when there are adds shadow priest dps goes up tremendously.
If you look at
realistic dps the charts change a lot. The difference then becomes 19k dps between shadow priest (#22) and arcane mage (#1). If you are outdpsed by every class by 20k on single target you are indeed doing something wrong.
Again, I don't look at WoL nor SimCrafts. I look at my own recount. My raid mates were doing slightly less dps than me in Cata, or the same. Now those players are owning me at every single fight, but 2 or 3 bosses with multiple targets like Emperor, where I do not shine, I'm just one more.
I think people forgot that you are raiding as a team and not everyone needs to be doing exactly the same dps on every single fight. There is also no thing like perfect balance, someone has to do the lowest dps.