When it comes to Soviet M4 it is already has smoke over other Soviet tank and it is also easier to benefit from radio net than USF one.
The advantage that T70 can give Radio Net Bonus to Vet1 M4C is easily more than outweigthed by the mechanism that you will put your vetted light vehicle / light tank crew in your newly build 76mm at USF. Thus having an instant Vet1 - Vet2 medium tank. Since Vet2 is the true powerspike of 76mm, this is way more valuable than having a slight advantage at Radio Net if you kept your T70 alive.
You do remember that the doc M4C Sherman come from has con repair kit. And now that even penals can repair, Your entire army not just the 2 engines can repair the tank, it already has access to versatile repairs as you can you use your front line infantry.
Only that it puts your Cons/Penals at combat/caping downtime and costs you a whole commander ability. Vehicle Crews have such a high value (crew swap, repair, critical repair ability with vet, repair of other tanks/structures, caping...). USF tanks have the advantage of their vehicle crews factored in their price and combat performance (thats the reason USF tanks are less meaty). So granting M4C to soviet army with the removale of vehicle crews at same cost is obviously a reduction of the overall tank performance/power level. The nerf to standard shell RoF (which was mainly aimed at USF mechanized Company) just took it over the top. KV-1 and T34/85 murder infantry and are still doing fine at AT performance, M4C just can't stand up to that since it's stronger HVAP penetration doesn't make up for it lower survivability and worse AI performance.
So either soviet M4C needs a price reduction or some additional tweaking.
One way to buffer the difference to USF version could be the repair ability for M4C which should be paired with removal of Con/Penal repair kits. You could give Land Lease either a further UKF/USF unit or a medicore offmap instead. Alternatively you could just swap the repair kits with the M3 repair truck that is somewhere in the files (you can spawn it at cheatmod). That way you would have another thematically fitting commander ability at Land Lease which acts as a mobile repair unit which can be moved quickly near your M4Cs.