Priests have a very unique place and issue in pvp currently. Alone a shadow priest is nothing much, we have very minimal mobility, a general lack of consistent burst damage, and a limited kit of cc and defensive CDs. What makes us strong in pvp is our incredible versatility and group support. We have unique cc options and a series of extremely clutch defensive options that makes void shift nearly uncounterable.
Despite this placement in pvp we lack one thing many other ranged classes don't: a slow. We are exceptionally easy for melee classes to train and are completely reliant on other classes for peels. While I appreciate classes needing counter play, and shadow having weakness to melee is a good example of this, there is also a certain limit to how weak a class should be.
I would like to suggest that rather than reduce the cast time of mind blast, mind spikes would cause a movement speed reduction to the target. Preferably a small percentage with a short duration, but that would stack 2-3 times. This would help give a small counter play to melee classes without being overly strong, and would keep us from being insta-gibber by high mobility melee. Alternatively adding an interaction with mind flay to increase the slow or provide a more consistent root to provide us with a little more self sufficiency in pvp with the advent of the large nerfs on the horizon.