As previously discussed ( http://us.battle.net/wow/en/forum/topic/2065568025 ) we're currently planning to put out a couple hotfixes specifically for Arcane mages. Likely today.

• Arcane Blast base mana cost from 7% to 5%.

However, with the above change in mind we are also going to nerf the Shard of Woe trinket to halve the mana savings for Arcane spells only. It's still a fantastic trinket for Arcane mages even with the reduction.