I was just thinking, if Lava Lash scaled with spell damage then your AP would be wasted on this attack, you would gain only 1/3 of your AP for damage, UR would not affect Lava Lash nearly as well, and Elemental Devastation would not increase it's chance to crit.
As it is now it scales much better then if it was a elemental spell that could proc Elemental Devastation.