Indeed, at lvl 80 around 5k gearscore I finally got dual spec and tried something else than frost, hence the arcane offspec.
With arcane, the mage got maybe about 1k more dps than frost, so went from 5k to 6k, or there abouts, but instead of having the unlimited mana as frost, the mage would go oom at least twice per bossfight, using mana gems and evocate etc every cooldown. And that is really not something I want to have to deal with/worry about when multiboxing.
And until the gear started getting up there, the frost mage was usually topping dps, closely followed by the ele shaman, which eventually gained a lead on the mage as her gear improved.
Not sure how much dps my shaman did last time I ran her solo in a raid, so I can't quite compare with how the mage did when I ran her solo arcane in a raid, since I wasn't running recount either of the runs.