Currently, it's optimal to get Sneak 5 on every character if you want the easiest combat experiences.
Do you actually play this game or just theorycraft?
Even with Sneak at 1AP, here's how it breaks down...
Build investment: 15 ability points in Sneaking, 1 talent in Guerilla
AP investment: +1AP per attack, because sneaking
always breaks after you attack. This varies widely based on the type of attack used, it could means your attacks are using 50% more AP, it could mean they're using 25% more, I guess with special attacks like Flurry it could be as low as 12.5% more, although I'm not sure whether Flurry breaks Sneaking before it hits or not.
Situational: One doesn't simply enter sneak mode. You need to get yourself out of vision first, which often means spending even more AP on movement in order to get outside of vision cones and/or other setup (such as creating smokescreens or inflicting the Blind status). So 1AP is the absolute minimum AP investment involved.
To make it even more situational, missing makes enemies turn around. What this means in practice is: the harder the enemy, the more AP you need to spend to get the Guerilla bonus. This makes Guerilla a "win more" mechanic; it's most efficient against the easier content which you don't really need it for.
In return for all of that, you get double the damage. Now if you're spending 50% more AP to get that double damage (daggers), that's 33% more damage per AP; if using two-handers (least "more AP" for default attacks), that's 60% more damage per AP; if you're spending 12.5% more (which is best case scenario), that's 78% more damage.
Now let's compare that with 15 points in One-Handed (or some other similar weapon ability) and Anaconda as your talent. That's 60% increased damage. Assuming you had 25% in increased damage beforehand (say, 15% from Leadership and 10% from a +1 boost), that's 48% more damage (in other words, a 1.48 multiplier). It doesn't increase your AP expenditure at all, and it's not situational at all.
The comparison comes out roughly equal.
Although Sneaking doesn't really get its big payoff until it hits rank 5, once it does it provides roughly the same benefit as an ordinary, boring weapon talent. That's not overpowered at all.
If there is anything exploitable when it comes to sneaking, the only thing it could be is: having an easier time maxing out Sneaking using bonuses from gear. It's normally difficult to track down items with +1 Bow or +1 Two-Handed (and especially +1 T-ability), but it's much easier to get +1 Sneaking on multiple items. There is a chance the item affix is too available. However, I would view such an occurrence as an itemization issue, not as fundamental Sneaking balance issue.
