First things first : Mind control is overpowered, and will be fine at 10 mana. I agree with this change.
Mind control in design SHOULD be a niche card, run only when the meta is very late game-centric.
That said, priest as a whole needs several tweaks and re-works. Even with the overpowered state of mind control, priest is one of the weakest classes in the game BY FAR. Priest has not won a single tournament so far except in Artosis' blizz-con deck rotation and the competition was popular players not skilled players.
Priest is very weak to classes with cheap removal who can establish board presence very easily - Priest cards rely on having board control without actually offering any way to reliably establish board control in the first place.
1. Pain - very good card but it costs 2 mana which means in early turns you give them initiative back when you cast it, surrendering board control. It also isn't affected by spellpower and thus can't be scaled up to take out bigger minions.
2. death - very powerful card but is a dead card most of the time due to the fact that low end neutral minions are more viable in competitive play than high end ones.
3. mind control - very good card except for the fact that with the way priest is designed, by the time you get to turn 8 you're probably already well on your way to losing if you haven't already.
4. shadow madness - very powerful card but is super situational. This card leaves room for the enemy to play around it. Sometimes the enemy just randomly wins the game because so many of the priest cards are dead cards until the perfect situation arises (which a skilled opponent will try very hard to not let happen).
5. Holy nova - oftentimes this card is too little, too late. This type of spell is put in a deck for the damage, not the heal, causing spells like consecration or lightning storm to be strictly better since they can be casted and comboed with at earlier points in the game. Most of the time by turn 5 2 damage isn't enough to make any significant come backs.
6. Other classes get things like backstab, soulfire, lightning bolt, rockbiter weapon, fiery war axe, stormforged axe which pretty much gaurantee board control unless the opposing player retaliates with similar low cost removal spells. Priest has no such spell, pain is the closest and it costs 2 mana compared to 1 or 0 which means you oftentimes cant couple it with a minion in the same turn like other classes can. This means priests always lose board control early on, and with the efficiency of buffer minions and the initiative advantage this almost always snowballs into a quick loss for the priest player.
tl;dr : priest cards cost more mana for the same thing.
7. The Priest class specific minions are garbage, so every other class has access to the same or better quality of minions that priest has depending on their class specifics (see S1:7 agent or flame imp). Lightspawn is gimmicky and bad, northshire cleric is gimmicky and bad, velen is too slow, temple enforcer is too slow and assumes board control (despite priests inability to establish board control). Cabal shadow priest is the only good priest minion and it is put in the very competitive 6 mana slot with cards like holy fire, sunwalker, argent commander, temple enforcer. This brings me to my final point.
Priest minions and spells need to be re worked to fit into smaller mana slots so that priest curves are more balanced and are better suited to establishing board control - temple enforcer should be a 3-3 for 3 drop that adds 2 HP rather than a 6 drop that adds 3. Cabal shadow priest should be a 5 drop with the same stats and effect. Smite should be 0 mana to be competitive with backstab and soul fire. Holy nova should be changed to either 4 mana or changed to deal 3 damage to everything. Holy fire should be rebalanced to be a 0, 2, 3 or 4 mana spell.
Maybe not all of these changes should be made as they are all vast improvements, but they are suggestions that would improve the class' ability to fight over board control rather than be forced into lying down and taking it like priests do now. That is the essence of what I'm trying to convey - board control is everything for priest and currently it's not even a fair fight compared to classes like warlock or rogue or shaman or mage.
TL;DR: nerfing mind control is fine, just buff other stuff that will give priests a fighting chance.