When I single box, I raid on my resto druid or arcane mage.

Haste is okay, but is mostly useless after your AB cast time is already at 1 second which is very easy to get with 4-piece tier 11. So I usually stack hit to 17% > mastery > crit > haste.

But raiding is of course a very different environment than multiboxing with 5 mages. With no tank, of course you will be wanting maximum burst and mastery won't take as much effect, and you need a much smaller hit cap.