Vs Cons you're going to win more at pretty well all ranges but long
Vs Riflemen you're going to trade very, very cost effectively as they approach (Assuming you aren't running your troops at them like some sort of madman)
Vs Tommies you can actually win close up, you get to take 3 models worth of casualties before your "main" dps takes a hit. Green to green at mid is still going to go to the Tommies though.
See, that's sort of the thing with that upgrade. Volks become, alone, capable of fighting every basic infantry the allies have. Volks having 3 automatic weapons reduces Shock Trooper's effectiveness as well, especially considering MG34s will be stock. And besides Volks already, especially with vet, hold their own quite well without any AI upgrades as it is.
If Volks aren't going to have Panzerschrecks (which they really shouldn't've), they should just have their basic rifles. If they're going to get an assault gun package it should be MP40s for all of them. Sure it'd make them clones of vCoH, but there's only so many German WW2 weapons to select from. And besides, it was a good design.
StGs are way too strong for all of five volks and splitting the squad up is extremely inelegant. The currently proposed method makes the upgrade either a no-brainer or a waste of munis, instead of a solid strategic choice. It's just not conducive to balance.
And this is all not even considering Sturmpioneers already have a set of StGs.