With a 3 minute cooldown I usually wait a few minutes before I do a hard boss if I need every cooldown and trinket. I like blowing mirror image on the entire run because the cooldown is so small.
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I haven't tested fire but frost seems to be near impossible to be effectively multiboxed. So i switched my mage to arcane as it seems to be the most efficient one.
I have a castsequence macro for her with Abarr just to reset and conserve mana, but i dont like to use that... Therefore my mage only casts Ablast now with apower, gem and trinkets... With this i get around average 9-10 k dps with busy fights, 13-14 k with fights where u just stand still.
Speaking for heroics i never had any mana issues.
For longer fights as i lke to raid old content occasionally; i can use a mana pot, evo, hymn with priest or innervate and have no issues whatsoever... She can sustain her dps with the help of the group...
If you are doing heroics and old content like me; mana should not be a problem and it may be wise to not use a castsequence for dps efficiency.