The regular Stug already counters light tanks. The Stug E not so much, but that's not what it's there for. I'm not really sure what to do with it if the regular stug were to get some better AI ability (again, via a shell swap like the M4, not by buffing the regular attack. I feel I should stress that highly)
Let me rephrase: do you think stug ought to be the counter to light tanks? i.e. it's what you build due it's position in the tech tree and ability, or you think that the stug *can* counter light tanks.
I feel that it's position right now makes it a poor counter to light tanks unless they're massively overextended. There's little reason not to build a p4 instead unless you're expecting a heavy tank soon as it's got a much easier time chasing due to it's turret, and can't get juked, or even circle strafed.
I could see that changing were the Stug in t2 though. My instinct is that it would get unlocked at battlephase 2(t3), but honestly I don't really see it causing any problems as a default t2 option.