So after discussing with my elemental shaman friend, I've come up with two ideas that could fix this problem. But I don't know how this will reflect in pvp.
1- Allow lava burst to scale with crit, like chaos bolt. This will solve our crit scaling problem
2- Double the cast time of lava burst but also double the dammage it deal. This will solve our haste problem. But it will affect our mobility, but I'm ready to loose that in exchange to stop benching for the last heroic bosses. (this is not a buff for low geared shaman, but its a buff for good gear shaman that can cast lava burst in less then 0.6 seconds)
What do you guys think?