Amrûnril
Hero
This is not helping your case ... at 9th level you are using a resource to deal slightly more than the at will damage for the worst non-caster in the game.
My point is that sneak attack is an ability designed to deal damage to a single target. Fireball deals its damage to every target in a large area of effect. If that damage is even marginally competitive when hitting a single target, then the ability to deal it simultaneously to two or three targets (let alone the half dozen or more you can hit in ideal scenarios) is incredibly powerful.