It does add damage. If you use sneak to hide from enemies and therefore be unattackable, Guerilla will increase the gain, because you will be able to hit even harder than you would be able to do using this tactic anyway. That is the meaning of synergy between Guerilla and Sneak abuse.
That's an issue which should be handled with AI improvements, not damage nerfs.
I think you're vastly overselling the value and effectiveness of Sneak. It can be broken if the AI decides to look around. If the AI decides to attack another party member, you can be spotted inadvertently. If the battle produces surfaces or AoE's, that can force you to move and lose the chance to sneak-attack. If the AI doesn't see you, and decides to attack another party member, they can move, and so you'll either have to break Sneak and lose the Guerrilla bonus, spend extra AP moving behind the target anyway (negating the bonus damage per AP), or break sneak, move and enter sneak, costing another AP again. I'll have to test to see if Sneak gets broken if you get hit by an AoE, I don't remember. There are many ways for the perfect plan of "Sneak at the end of turn and get free damage" to fail.
Guerilla-Sniper is probably the best lone wolf build to abuse since you can actually go all-out guerilla tactic.
Guerilla enhanced attack to start the fight, kill something, flee. Repeat until they all die just like the name suggests.
In my opinion, if a player wants to do the same Snipe + Flee over and over again for every battle in the game, that's fine. They're only going to be taking away all the fun from themselves and boring themselves to death.
And if necessary, that exploit can be fixed too. Maybe something like "if all party members leave a certain radius of an enemy encounter, their health and armor could be restored to full", so as soon as Snipe + Guerrilla fails to one-shot an enemy, the snipe + flee tactic fails to work. That's just a possible example.
