Soviet mortars are especially bad against OKW due to that factions lack of support weapons. At least Ostheer has some static targets for you to shoot at. Besides, you need 3 soviet mortars to match the rate of fire of 1 Ostheer mortar, so I doubt it would become a problem.
I don't think the Ostheer mortar is OP. I like that it is a highly effective unit that is often worth getting over another grenadier squad.
Likewise, I don't think the MG42 is particularly OP (maybe ever so slightly).
What I want is for allied and OKW support weapons to be on the same level of viability. Maxims, DHSKs, LeIG, 82mm mortar and the HMG34 are some examples of units that are completely outclassed compared to the Ostheer equivalents. To give the other factions the same incentive to use combined arms over infantry spam, some units need to change. One way to do it by improving the units themselves, the other is by keeping their performance and lowering their cost.
^^^ This, there isn't any need to nerf Ostheer. Ostheer should be the model for every faction. Support weapon play is still dead when it comes to OKW.