If a player decides that they don't want support weapons, they are giving up most of their early game power. Grens simply cannot clear sandbags and buildings easily, nor can they hold off rifles. Its a choice that they make. ( see my post one page ago).
Giving grens a 5th man and 280 mp is a terrible idea. Grens already have higher dps than conscripts at close range. The only reason they lose is because the cons have more models. 5 man grens would beat cons at all ranges, and directly feed the grenspam you just said you don't want. All you would be doing is giving ostheer great early game infantry, exceedingly cost efficient support weapons, and 0 drawbacks.
Players will decide they don't want support weapons based on cost efficiency, if the MG42 and Mortar cost a lot people will just get a few grens out and rush t2. It would make Ostheer in smaller game modes harder to play and still piss easy in team games.
It would also make reinforcing more hellish than it already is, to many model losses can drain away all your MP early game very quickly making come backs hard.
Personally I think the 4 man squad nature of Ostheer makes it way to vulnerable to squad wipes (USF shouldn't have 4 man squads either), and paying more MP for the same DPS but more durability sounds like a fair trade all things considered.
But ultimately I think the best solution is just to give the other side buffs instead of giving Ost nerfs, the reason Ostheer is so good right now compared to the other factions is that they got the most Alpha buffs of any faction.