It's one thing to buff underused units so they can perform their intended role, look at the good changes to previously underused units such as Brumbarr or SU-76 as an example.
The difference is, that the SU-76 received the necessary buffs and changes to make it perform well in its intended role - as a cheap, mobile AT gun, meant to stop Panzer IVs.
The Stupa never really had a role it was meant to perform, at least not since Relic changed its projectile profile to artillery, effectively killing any chance this vehicle ever had. In its current state, they could lower its cost to 200MP, 100 Fuel, and it would still not be worth it, simply because at that stage of the game the vehicle is unnecessary and completely without purpose.
But that is not the problem of the vehicle. The problem of the Ostheer as an entire faction is, that it lacks initiative. It doesn't matter how their infantry and vehicles perform in direct comparison, what matters is that the player has a way to take initiative and through superior play dictate the flow of the game. And Ostheer can't do that. This is not a problem of how survivable Grens are, or how much manpower the MG 42 costs. This is a problem with the basic faction design - and no, it has nothing to do with asymmetric balance. The credibility of that argument went down the drain, when Relic released the Brits and showed us that they actually can create a faction that is defensive in nature and still has the ability to take initiative.
The effect this has in the game is that your opponent dictates when, where, and how battles happen. When playing Ostheer, you have to react. The entire faction design forces you to react, never being the acting element. All your weapons are designed with the other faction in mind, not your own (which is true to some extent for all factions, otherwise balance would be impossible, but it is the entire basis of Ostheer balancing since the WFA were released). You won't find a single element that is balanced around making it fun to use and balancing the other factions around it later, they all are done with the thought of how they will perform against the enemy first and foremost in the mind of the developer.
Ironically, the only exception to this rule, and the reason why I am even writing about it here, is the Stupa (or Brummbär, call it as you want). And the way the other factions were balanced around it was to make it entirely pointless. Which leads us back to the basic premise of this faction...