imo the set bonus for elemental shaman tier 10 should be something like:
2 pc : Your Lava Burst extends the duration of your Flame shock on its target by 6 sec.
4 pc : Everytime you cast Thunderstorm your spells deal 10 % increased damage for about 7/10 sec.
blizzard idea of 4 pc bonus should replace the current 2 pc and i like the idea given to other classes like mages to give a flat damage boost after using a cooldown, in this case (shaman) its a smaller boost for a shorter duration to compesate for the lower cooldown on thunderstorm compared to mirror image,