So if you have the fortune to fight against the Blood Elf mage(i dont know the horde equivalent) make sure to spell steal the haste buff. If you also have the fortune of being arcane, pop IV and get your missile barrage proc. Lastly have your shaman in group pop hero.
When the stars aligned for me I let my arcane missiles fly. Casting time for all five ended up being ~.6 seconds.
It's worth doing at least once. The missiles are nearly on top of each other, you'll probably get a few /w's asking what the hell just flew out of your hands.