I run 3x arcane mage for my heroic group and 4x mage for PvP group. I've run several different configurations of arcane and for me I just spam arcane blast until the boss is dead. If you don't do enough dps total in the group that may not be an option but i would be willing to be /castsequence is hurting your dps.

Casting arcane missiles will hurt your dps unless you proc missile barrage.

you could try something like
/cast [mod:shift] arcane missiles; arcane blast (not sure on syntax)
that way spamming that button will spam arcane blast and basically you hold your mod key and press button to cast arcane missiles and clear your arcane blast buff.

When I ran this type of setup on my arcane mage i used mod key as kind of a mana throttle. If my mage had less mana % then the boss had health % i started cating arcane missiles every 4th arcane blast / missile barrage proc etc.

My 3 mages can kill most heroic bosses just spamming arcane blast and not run out of mana. Granted they have 24k mana and do 6k-8k dps.

Edit: I just thought I'd point out that my mages have absolutely no raid gear, all their gear is heroic / triumph gear.