The SU-76 is simply superior after vet 2, it does more damage.
Its only +20 damage, makes it 140 dmg per shot, only good vs lights, like 3-shot the 400hps.
Its far pen now is 160, no guarantee to pen ost p4, hard to say it can be effective against mediums when not in numbers. As for ostwind, it may simply lose to it in a duel.
puma with vet is much better with maneuverability and sight range and damage and survivability. su76 has good pen and range, but thats what a td without turret and armour should have or it will never be used.