Stugs can be really deadly against Soviet T3, especially when used to back up a P4. They're not as good vs USF though. Jackson just demolishes them. Luckily the PaK40 is extremely good vs USF.
But yeah Su85 and Panther could use some love. I'm not sure about the Jp4. It seems to perform much better than the SU85, I think a lot of it has to do with the insane frontal armor - and it gets some amazing vet stuffs.
As for improving USF late game... hmm... maybe tone down rifles a bit at vet 0 but improve their vet bonuses to compensate. Essentially they'd be worse than they are now at vet 0 but once they get to vet 3, they'd be stronger than current vet 3 rifles are. I think this could go a long way in helping Ost and OKW vs USF early pressure, while also making USF more competitive with the late game infantry of both factions. I don't have any specific numbers but it is a general idea.
I don't know if the US fielded any AT guns bigger than the 57mm but adding a doctrinal "heavy" AT gun might be interesting, if there's a historical precedent for it. I'd also lower the cost, or improve the performance of the pack howi. It takes too long to pay off from my experience in 1v1s, although this would risk breaking team games... maybe...
A doctrinal USF heavy tank could be nice. T-28 or Jumbo maybe? I dunno, I'm not super knowledgeable on the history side of things so maybe some one else can suggest some better heavy tank options.
Overall I think lethality is actually slightly too high across the board right now. Obers, Fallshirms, BARS, M1919A6 and some tanks being the biggest offenders. I'd like to see the power level of the game drop a bit - especially mid/late (but keep USF tanks the same) and see where things stand. Nerfing USF early game, while nerfing everyone else's late game would effectively buff the USF in cases of a long game.
The US had the M5 3-inch gun:
http://en.wikipedia.org/wiki/3in_Gun_M5
Other then that there was 90mm AA gun.
As for heavies the the US had none. They had a few projects with the M6, T95 and the T29, but none went into mass production. Their best tank was the pershing which was really only a advanced medium tank more like the Panther and only 20 saw combat.