I would not be adverse to a slight increase in ZiS RoF.
I wtill think Barrage is a problem, but I think the importance of MP based effective AT is more important than that, regarding to this.
Don't increase the Zis RoF when barrage still exisits. If a barrage shot hits a tank, it's hitting a tank "as normal dmg". I feel barrage at 60(?) muni is a fine cost.
Frankly, Id remove it from ZiS. SU76 carries it for free. Its a double redundancy to have it on ZiS as well, especially considering Ost has no equivalent on anything.
ZiS is T2, Su76 is T4. Yes, both are similar but, this can be taken as a "core tactic" for the soviets.
Spam cheap, crappy inf (conscripts, penal squads)for sight and have ZiS' & su76's Artillerly from a distance.
The fact that it's redundant is a BENEFIT not a negative. ZiS being versitile at anti-inft (for muni) and anti-tank makes it great. Then add wheels and fuel and you have the su76. Add more and you have su-85.
Ost doesn't have an equivalent because they are more moblie and have more dmg/ are tank-based/vehicle-based playstyle. However, you can make pgrens get Schreks (similar to anti-inf & anti-tank capabilities for muni), and have earlier & cheaper access to better 'overall' vehicles (flame-halftrack & scout car [especially with upgrade])