As it's currently written I believe it's overpowered, and would never allow it in one of my games. It's a feat with a benefit that scales with level (as you get more sneak attack dice, the bonus damage from the feat is increasing as well).
I think it should simply grant +1d6 sneak attack damage, with a prerequisite of doing +3d6 sneak attack damage.
I think it should simply grant +1d6 sneak attack damage, with a prerequisite of doing +3d6 sneak attack damage.