Why is it that sap, a crowd control effect that rewards you for being in stealth, and punishes you for being in combat near the target, breaks stealth when you use it? Stunning Shield from Vanguard doesn't even break stealth, why should Sap? It would be 100% more worth a feat if it could be used without breaking stealth. at the state it is right now- even if i were to find a party that were conscientious of the fact that i am putting a CC ability on an enemy that will break if you sneeze near it- i cannot imagine that it is terribly useful, except maybe for taking a single archer or caster out of the fight after the party has already aggroed.
I have two suggestions - both of them would likely benefit from the removal of the damage dealing aspect of Sap - which in reality may entirely be the culprit from the mechanical side of things.
One, make Sap inherently not break stealth. I would go so far as to say you could remove the bonus duration for attacking from stealth and it would be immeasurably more useful.
Two, give the stealthy enhancement trees (Ninja Spy, Assassin, Deepwood Stalker) a way to use abilities/attacks without breaking stealth. As long as dealing damage to an enemy puts them on alert for stealthed players i cannot imagine this being a game breaking effect.
In general i think the entire stealth aspect of the game needs a bit of a retuning - building a character around utilizing stealthy tactics (like sap) often is either entirely meaningless as you are grouped with people rushing through dungeons giving you no time to do your stealth thing, slows down a party so much that it isn't even worth bringing a stealth character around, or requires you to play solo. And even if you partake in the latter two, you are rewarded significantly less for being stealthy in a quest than you are for killing anything that moves. Perhaps instead of a stealth xp bonus for not killing anything in a dungeon (which is often impossible due to how many things break stealth) have a new XP bonus attributed for killing x creatures in a quest without having alerted them first. This in tandem with the two above sap/stealth changes could make it so that bringing in a stealth character to dispatch as many kobolds in a group before the kobolds notice, and the rest of the party can rush in afterwards and not feel like them standing just out of sight wasn't a complete waste of time.