The IR StG is a downgrade from the MG34 in almost every way, it's best bonus is being able to clear units out of cover which OKW lacks pretty hard since they have exactly 1 good indirect fire unit.
And Grens are not fine, they lose out to even cons at an early stage in the game due to being 4 men and lacking enough long range DPS to damage a squad enough so they can't just get in your face and wipe you.
My solution to the Obers problem has already been stated, take away the MG34 and give them 2 normal StG 44's. The IR upgrade would just give them better range and ability to negate cover. I also think that Shocks and Thompson should get a medium range buff and a close range nerf.
I dont think IR STGs are downgrade at all what so ever. Obers with these upgrades melt shocks, and all before it in seconds. The whole point of this upgrade is to use this commander in close quarter maps and not like steppes.
I suggest you watch ImperialDane's video where he discusses the damage distribution with Grens (4 man) vs Cons (6 man) in detail.
Solution to Obers = munition upgrade to their lmg like every other unit in the game. OKW needs more muni dump than just shreks.