The agi player is not to supposed to only take one of Gunslinger and Hidden Strike. He either specs into Gunslinger+Run and Gun OR Gunslinger+Hidden Strike. The latter multiplies the damage you make on 6 of 7 days.I do not think this is necessarily a good trade off. Every point spent on sneak attack is a point not spent on combat skill that increases damage.
In Agility attribute you can either take Hidden Strike or Gunslinger. For the sake of argument we're not entertaining "What if they take both?", that'd lead to a never ending "they could've taken these skills instead". Incidentally, they both cost the same as far as skill points go
Hidden Strike grants 200% extra sneak attack damage on top of the standard 50%, so you're dealing 350% damage per attack
Gunslinger deals 50% extra damage with 100% extra damage every 3rd shot, so about 115% extra damage per shot for a total of 215% damage per attack
In order to compensate for the loss of Hidden Strike, you'd need to gather more than double the usual bullets, and considering you'd still need at least 1 bullet per zombie to kill them anyway, I don't think Hidden Strike is good enough.
Not to mention the distinct advantage faster reload brings, which would up the damage dealt per second
but as you say stealth may get buffed, so they recognized there's a problem
This is similar to a perception player speccing into sniper+penetration and a miner speccing into miner69+motherlode.