I still think the win/loss % speaks for itself and nothing really needs balancing... if everyone had perfect aim, shotgun would be overpowered. hell, if everyone had perfect aim, the LMG would be overpowered. maybe for that reason the SG IS overpowered in competitive play, where the players are more serious/skilled - I wouldn't know, I don't play competitively. all I know is that with average skilled people, like myself, sg rushes are not instant game winners. in a 7 v. 7 game, most sg rushes go like this: 4 to 6 marines w/ shotguns head toward the hive. I'll leave out the rambo factor and assume they're actually sticking together. 1 or 2 get snuck up on while en route and bite the dust. the aliens who spotted them alert the team, and a couple head back to the hive. the 3 to 5 or so marines left start unloading on the hive, often getting picked off while reloading. if the alien team is actually cooperating, enough of them would be back at the hive to finish of the 2 or so remaining marines, who cannot gun down the hive in a nerve-wrackingly short period of time.
I still say stealth is the counter. you could just as easily say cloaking is overpowered. yes, there are counters to cloaking. a comm who pings a lot and builds strategic obses (rare), or just getting MT... but cloaking used intelligently (like not moving when the marines are reasonably close) can nail even the best unsuspecting players...
I'm not honestly arguing that cloaking is overpowered, just saying it's 1 possible counter for shotguns. yes, it's possible to have a successful shotgun rush - usually when half your players are dead and they're already in the hive. yes, it's annoying to be gunned down point blank in one shot. these are just products of player stupidity and bad luck, though...
only time an alien team I'm on gets pwned, by shotgun rush or otherwise, is when it's obvious the other team has better players. otherwise, people know where to be to stop a rush, and how to come at the rushers...