M20 would automatically become more reliable by the nerf to heavy TDs. No longer will the minelayer/scout unit of the USF be blown up just because an Elefant/JT stared at it for a second
I think you are overestimating the M20 here. OK it will be good to not having it one shot by super TDs but one shot and you need to withdraw to repair. I don't know how you play USF but you already need to manage a bunch of units that can be kill in two strikes, adding the M20 on the melee will not solve the problem.
I repeat myself:
1- The problem between USF and those big cats isn't that you can't see the elefant or the JT, its that they shoot at you once and you need to retreat and repair.
2- You can't flank them, sometime it is because of the map, but let's be honest, it is mainly because you have paper made tank destroyers that can't flank in the melee on teamgame. there is always something that will kill them on their path.
3- You manage to shot at the ele or JT, good, sometime you penetrate, sometime not but what is sure is if the ele or the JT is ready to shot, you'll get your load of shit in return and the only way for you to shoot again is if it miss you otherwise have to retreat and repair.
4- OKW does have raketen and the strategy is even simple: they put their camo rake in front of their army, they rush you with some volks, you fight and if you forget your tanks cuz you are busy keeping your infantry alive, JT fire and Rake finish the job.
5- Best ele doctrine does have scope thus making your M20 completely useless. Reco+scope+222vet3scope + stukadive = your atgun are dead before your M20 even localizes its ele. And then your TDs are completely useless. You can modify the Stukadive, nerf it as you wish, it is not going to change anything unless you nerf the stuka dive to oblivion which nobody wants.
My advice: You're not going to change anything with the Elephant or JT by changing their stat values. Those units are made to be or OP or useless = look at how the ISU is useless today.
You had a interesting idea to increase their Popcap and I think its the way to go. Elefant and JT should be like the TigerAce, units that cripple your economy. Not at the same level of resource starvation as the TA but still, if you opponent resist you and kill your army around the Ele or the JT, you are fucked cuz you can't reinforce and rebuild properly.
This, in my opinion, should be the way to go. Those units are mean to be top performing in their area. Fair enough, but make sure others areas are negatively affected in return.