Well to be fair here, why would you want build obersoldaten without LMG, if by the time you can build them, they will face vetted and upgranned inf.
Only time I would ever do this is to get an Ober on the field so that I can research PA while they're walking towards the frontline, then the LMG upgrade arrives close to the time that they actually need to start shooting. On their own even vet II 4 man IS can outdps your face iirc.
How overpowering would they be if you could just buy them from the beginning of the game? I expect some of their vet 2 bonus might have to be pushed to vet 3 if that were to happen.
They would literally walk over any mainline infantry in the game at that stage. In any case Obers are meant to be this big anti-infantry specialist, the only T4 infantry in the game. They're supposed to be able to crush mainlines late game.