The fact is that a 264/277 geared ele shaman will do 2k less dps than a same geared arcane mage for example.
Let me give another example. Proffesor Putricide: Ok, a moving fight. Melee prolly having advantage because of melee range. A Warlock or SP puts his DoT's and probably chanelling spells. What about ele shamans? Flame shock > Lavaburst > Moving > Lavaburst > Flame shock > Moving. Not exactly like that but no elemental overload spam, no Tier 10 2nd set bonus. In fact Ele shamans can do a good burst when not moving. But we dont raid Hogger. I dont want to QQ hard but thats the reality. Ele shamans will do a nice dps because of no moving and maybe chain lighting spikes in melee range. But what about other fights? Proof that Blizzard understand that problem is: Spiritwalker's Grace.