This won't go live, or if it does, it won't stay live for more than 24 hours.
What they think their goal is:
"Shadow Priests who want to focus on single-target damage can aim for Mastery in their gearing; or, when multi-dotting is more valuable, prefer other secondary stats over Mastery."
What they think will accomplish this goal:
"Shadow Priest’s Mastery (Shadowy Recall) has been replaced with a new Mastery: Mental Anguish.
Mastery: Mental Anguish: Increases the damage of Mind Blast, Mind Spike, Mind Flay, and Mind Sear by 20%."
For this to be worth anything, this mastery needs to scale to like, 60-80% with mastery stacking - Mind Spike, Mind Flay, and Mind Blast make such a small proportion of our single target damage that this mastery won't actually be a single target increase unless these are massive % modifiers like this.
Also, I'm not sure if they are aware of this... but Mind Sear is a very potent AoE spell... on a single target mastery? /boggle
Mind Spike isn't worth casting unmodified in MoP, which means that it - proportionally - can't be any much lower in WoD - it's too shitty to have a spot in our spell priorities without FDCL/ToF - so it can't really be any worse than "not worth using". Given that, regardless of how they rebalance proportions - it's still fair to think of Mind Spike doing proportional damage to what it does currently.
Why am I focusing on Mind Spike?
Let's say Mind Spike deals 100% damage. Modified by FDCL, it deals 150%, and etc:
100% * 50% (FDCL) * 15% (ToF) * 40% (Clarity of Power) * 80% (Mastery: Mental Anguish) = 434.7%
Which is to say that Mind Spike under ideal conditions in WoD would deal ~435% of base damage. Even if the base damage isn't worth casting under any conditions (as it is now) - dealing over four times the base damage is liable to be overpowered. Further, assuming the 3% bonus critical damage Meta Gem still exists in WoD - a critical Mind Spike would deal a whoping 882% of base damage. Doing nearly 9 GCDs of damage in a single instant cast proc will never be balanced - let alone stacking up a couple of those procs and dropping them back to back for potentially 18 GCDs (a full minute of mind spike baseline casts) in 2 GCDs.
And that's not all - what it does to Mind Blast is similarly unbalanceable.
Mind Blast = 100%
100% (baseline) * 1.15 (ToF) * 1.4 (Clarity of Power) * 1.8 (Mental Anguish) = 289% of baseline
A critical Mind Blast under those conditions would deal 587% of a baseline non-crit.
That is simply unbalanceable - it's just too big of a range - and the argument that it would be hard to juggle ToF and Clarity of Power and FDCL or DI - or that we have to stack mastery, or use some obscure mastery proc (or on-use) trinket to make it happen - won't appear the veil of tears that PvP Spriests will cause with a mastery like this, talents like this, and class design like this: something has to give.
So, don't get your hopes up - because while they apparently have no idea what they're doing with mastery, and they don't know what Mind Sear does - they will know that somethings wrong if this went live