Originally Posted by
godofslack
Frost mages are (currently) not viable in 5 mans, the main benefit comes from not needing to recover mana many times a fight. They also have a massive ramp up time (due to the random nature of their DPS) and are very close to the current ret pallies (long ramp up time, low burst, nearly infinite mana).
On the topic of mastery I can't begin to tell you, but it is worth any extra crit/haste into mastery as the frost mage mastery is an HUGE increase to DPS, honestly crit isn't even that valueable.