Shadow is not top dps. Spots vary from fight to fight which is the way it should be. There is no real gap between anyone in the top 5 spots on most fights by the end of an encounter. Hunters do ridiculous damage in the first 20% of a boss where-as warriors do insane damage toward the end of an encounter. Fire mages / warlocks / Boomkins do crazy burst aoe. Boomkins / Shadow priests do crazy sustained multi-target dps. Warriors/Hunters are very good at swapping targets to adds that need to die quickly. Rogues do amazing single target damage are good for kicking and control. etc etc etc Every class has there viability and the top of the meters are held by classes that don't have "responsibilities" during encounters such as dispelling or kick rotations.
Shadow is in a good place and hopefully this change to mind sear will make it worth casting in certain encounters like Chogall or Magmaw or Maloriak or Sinestra, but I dunno. Said it before that even doubling the damage didnt seem like enough. We shall see.