The StuG III Auf G (the "G" was the most numerous variant at over 7700 produced) mounted the same 75mm KwK40 L/48 that the Panzer IV (albeit a different mount, iirc). The gun performance would be no different that AH's Panzer IV.
Other comparisons to the Panzer IV:
Same speed at 25mph; the StuG III carried 54 rounds for the 75mm vs the Panzer's 82 rounds; the StuG III has one less MG than the Panzer IV; frontal armor is the same at 80mm with similar slope.
I hesitate to call it a legit TD since it was designed and use as an assault gun working hand in hand with the infantry. A legit TD is going to be designated as such as have limited HE. Even AH's resident TD really isnt since it has an abundance of HE available when it is debatable that it ever really carry more than 20 rounds of HE. It was always paired with 75mm Shermans with the much faster firing gun and far more effective HE round.
Regardless, I'd like to see it added once the steering is up dated.
