If I get caught with my pants down and a Sherman in my base its gg...
That's the same mentality with getting the shreck instead of the MP40s. In addition I am quite certain MP40 volks get chewed up badly before they can complete the charge to close range.
Even if you only got him to move I would say that the unit succeeded no? That wall was the only thing allowing him to destroy you so easily. On the open field your units would've been much more effective than his. An actual hit would've decimated those squads. You could've used it against his mortar as well once it hit the field, but you could'nt have known about that one before you purchased it.
In addition, correct me if I'm wrong, but I think the USSR mortar (un-upgraded) already does 1.5x damage in comparison to the Wehrmacht mortar