The OHK radius is increased by increasing the damage. It technically also applies for the normal gun, but for AT specialists this is negligible. The SU76 barrage at vet2 gets an OHK radius of 0,96 compared to 0 at vet0.
For the stats and cost, the SU76 is actually decent. In my opinion it is the same issue as discussed in other threads before: For its T3 timing, it does not fit the faction. Soviets need an AI specialist to counter their infantry weakness in the mid game. In the late game, they just have better options than the SU76.
The only point where I can see it used is therefore a last ditch effort to not be overrun if you lag behind in the game or potentially an Ostheer player spamming T3 units. But the last one you will only encounter after already having invested into T4, so you'll usually stick to T4 units as well.
Yeah if the su76 arrived earlier you when there are actual lights still on the field it would be an absolutly fine unit. Cus soviets dont only have a inf weakness mid game, they also are not very strong vs light vehicles early on. Both make the mid game harder for soviets imo. Both ost and okw have fast and strong lights with smoke in most cases.
If t3 is split and su76 and m5 arrive earlier it would give soviets a choice instead of always go t70. Not that this will happen but i can hope it will someday.