That's ~ 40-some damage at pretty high level in for 2 full round actions - amount that can be ridiculously easy done with sword even earlier, with often quite a few attacks in those 2 rounds.That includes sneak attack die? You're at least at 3d6 sneak attack by the time you get snipe, and crossbows do 1d8 to 1d10 damage, depending on if it's light or heavy, meaning you're doing a hell of a lot of damage with a single blow (2d10+6d6 at sixth level?).
So that doesn't really seem very broken to me, considering fuss required.