Destro has at least 2 GCD's any time they want to move, Rain of Fire and Conflag. There is no reason you can't just always sit on one conflag charge for movement. Also, shadowburn. For example Ele shamans only have Shocks and Totems to cast outside of Lava Burst procs so 2 GCD's isn't really that bad.
Right now if you ignore what people are doing with Soul Swap, and its clearly unintended damage boost all three specs are very close in most situations.