Make Wicked Sting a Passive or 1 / challenge minor action
Nearly pointless; you're nearly always better off going for an Off-hand + Main-hand combo (or better yet, Shadow Strike) when breaking stealth to attack. Doubly so when you get Stiletto. Only has limited niche use when under heavy pressure to attack quickly while stealthed (usually due to party members getting in trouble).
Just make this a Passive which increases the damage of your stealth breaking attacks by +2, and has them impose Weakened on a critical in addition to their other effects, or a minor also usable as a major, at the cost of reducing its use to 1 / challenge.
Wicked Sting - Passive
Benefit: Whenever you make a weapon attack while unnoticed, that attack deals +2 damage (or +25% damage, whichever is greater) and imposes the Weakened condition if it is a critical hit.
Wicked Sting - Minor or Standard Action
Benefit: Whenever you make a weapon attack while unnoticed, that attack deals +4 damage and imposes the Weakened condition.
Except it's inferior as compared to virtually every other option of its level; Shadow Strike, Twin Strike and Parting Blow (as a prerequisite for the extremely powerful Blade Dance) all are about unarguably superior to it. There is no internal balance between it and competing options.
I think Wicked Sting is fine how it is, and that the Rogue class, as it currently is, should be your benchmark to measure other classes against when determining tweaks for gameplay balance purposes. You have to choose to take Wicked Sting, so people who don't like it/don't think it fits their play style can take something else.
Further, I'll also note that whatever time pressure niche use it has is even further obviated by flat out superior limited use attacks such as Twin Strike and Blade Dance.