Cast sequence is shown to have a delay between casts - if you're using all 4 DPS as shamans its going to hurt you. I use a castrandom and it works perfect for me:

#showtooltip
/castrandom Lightning Bolt,Chain Lightning,Lava Burst,Purge

This works great and will only purge if theres something to actually purge (which if there is you almost always want it purged off anyway).

For bosses, I open with my tank then open my DPS with a special macro for just the start of the fight:

/castsequence Flame Shock,Lava Burst

Spam that until LvB casts and then just move to my castrandom. If I'm trying to conserve mana I have another key to just spam 4x LB over and over again.