Why does it have charge?
Most people have to spend a turn killing a board (Flamestrike ect), Then spend another turn adding units.
Priests can waste your entire last turn, put down a huge minion in a turn, and then attack ON THAT TURN ALL with one card. Be it 8 mana or 10.
This means mind control effectively gives you charge in a warped sense.
What needs to happen? The minion is freezed for a turn after taking it, giving you two turns to respond. So the priest also wastes his turn effectively just stalling the game out a turn which gives card draw options for his other answers in the deck. Sure, they are a card up on you but it doesn't exactly stop tempo this way and gives the option to still rush the priest down.
Changing it to 10 mana will only keep the rage and weaken a class for a stupid reason. 1 card should not be balance. You need to tweak minor things across the board for balance. Charge is worth about 2 mana (Argent vs Lord of Arena) so it pays for its self on any minion 3 mana or more. So don't even bother trying to cap it because most people will use it against a 5 mana or more unit anyways.
P.S: I have a 82% win rate priest deck atm without any mind controls. Priest is op without mind control and needs another nerf, but this is a good start to find out what the issue with priest is. (I say its injured blademaster because you can heal it without costing you a card, meaning it trades 2 for 1 or more much easier, with a power word shield cantriping, its like an OP yeti on 4.)