It's not any different than having infantry units using the same weapons, having different performance on different ranges.
Range on tanks is not hard. 40/50/60 with some unique cases in between.
You could give the Stug 60 range, but then you would need to drastically increase the cost or make it deal 120dmg per shot as the Su76. Any drastic change means a rework, an effort which i don't think is needed when this types of units just need a nudge here n there.
Best would be to overwork the vet system and add an update.
e.g. remove the vet2 armor and hp buff, instead make split some vet3 reload with vet2.
With T4 StuG get can make a upgrade (100mp and 20fuel) to get armor and hp, +5 range. (rename normal StuG to StuG F, upgrade to StuG G) in real live they change the barrel to a longer version, so it would be authentic too.
Edit, same can be made with Panther and Panzer 4. Maybe only a armor buff but split the vet-ability, so they become faster better. Better armor needs a upgrade. (to get an answer for the Panther-redesign and Jackson thread. ;DD)