You could discuss it instead of just bitching your opinion. I wouldn't call it extremely powerful though. 76mm shermans go down to panthers and you won't be able to field 76mm and weapon upgrades. The jumbo could be penned by the rakten, panther, ect. American tanks still suck even if they're good remember. Only the m26 would be that effective.
The 76mm Sherman is a very effective vehicle in the Soviet faction, I imagine this would hold true in the American faction as well. 76mm Shermans should be the staple of a Commander, not a 'filler' ability like you claim.
The claim to fame for the Jumbo Sherman was it's armour, why do you want the armour to be weak? The Jumbo, like the 76mm, would be another staple ability. It's strength would be his armour, and possibly a 76mm gun.
The M26 Pershing in this case would have to be the
staple of this doctrine. Everything in the doctrine would be a lead-up to this unit. The unit would likely be weaker than the Tiger or IS-2 to make up for the fact that the US Faction comes with the M36 Jackson.
I don't have replacement suggestions, but I could tell you this wouldn't work. Rather, all 3 of these would do better in separate doctrines. One with 76mm Shermans, one with Sherman Assault Tanks, and one with the M26. Personally, I feel like the M4A3E2 Assault Tank could easily fill the role that M26 sequesters want, without breaking the faction rule of "No Heavy Tanks".