I was under the impression that these units had a larger difference when it comes to firerate, thanks for correcting me. The firerate of the tanks was not my main concern however, but simply that both vehicles (one being a specialized version of the other) are solid picks and that given both options you don't always opt for the same one. This is not the case with a lot of commander tied stuff, examples given are svts/7men, 5 men grens/lmg, and shocks troops that has no comparison what so ever in the soviet tech tree.
The 85 isn't a specialized version of the 76mm it's just a bigger version, unless that specialization is simply "being better than the 76mm itself n every way" in which case I suppose that's true.
7 man however does a good job of being attractive over the svts by being ideal for recrewing on the field. 7 man makes merge infinitely more useful. The big problem with 5 man grens is that there really isn't a trade off at all. Sure you lose the concentrated DPS of the 42 but you get mobility and durability and even utility via free heals out of it. Losing something, like the snare as was suggested would tone the 5 man down enough to make lmgs a part of the composition instead of a replacement. And giving lmg grens a bonus against suppressed troops would solidify them as versatile defensive infantry which is much needed.