It's a nice idea but I think giving PTRS Penals more late game scaling (with a second +2x PTRS upgrade at T4) is better because then Soviets have good mobile infantry-based anti-tank. Dealing with Elefant/Jagdtiger + Panzerwerfers/Stukas in team games is so obnoxious because Ziss guns die to the Rocket Artillery and and SU-85's die to the heavy TD's. All Soviets can really do was ram cheese but now that's been heavily nerfed. Whereas both USF and British have much easier times with Bazookas and Piats as well as turreted Tank Destroyers that can flank. Having mobile squads with AT on Soviets gives a lot more potential and opens strategic options. A second PTRS upgrade at T4 allows for late game scaling without being too strong against LV's or Mediums in the mid game.
Because the main problem with PTRS as an AT weapon is reliability and micro. It's a defensive weapon that only punishes people who stay for a long time and are indecisive in regards of what they want to do. All other AT Infantry can just shoot a rocket and move while reloading. If for whatever reason the targets leaves range by the time the weapon finishes aiming you have to repeat the process.
What makes Penals threatening to an Ele/JT is their AT satchel not their PTRS.
I think there are 2 complete different issues.
1- PTRS doesn't make Soviets have a mobile AT platform in the same way all 4 other factions have.
2- Giving them another 2x PTRS will indeed increase their AT capabilities and make them not be dead weight.
I just think IMO, that Penals would had been much better as purely AI units with AI upgrade which can tech AT nades for making their satchel sticky and any early AT they required would be supplemented by the 45mm.
Also PTRS are not good enough against FHT, Flak HT nor Ostwind.