Every high level player i know of thinks the stuart and T-70 massively over preform. Same with the US Mortar. And the Stuart came out very early before the mortar was introduced. The biggest problem with the Mortar is that is wipes, without warning.
I don't think a single Bar is good enough. When you have to be so aggressive with USF (even now with USF late game is hard if your opponent gains an equal footing) you need a squad with A LOT of punch, against axis mgs sometimes you will only get one or two squads though. Those squads you do get though need to be able to carry on doing damage. If you make it one bar you are likely to simply not do enough damage. Mix in Obers and pgrens and the rifles cant win.
One of the biggest problems with facing USF is that they can be balls out aggressive and flank your mg/mg's which makes OST main way of defeating opposition infantry null. With a slower unit thats only effective at long range you allow for OST to react with their mg. Tone down the effectiveness of the lmg and reduce the price a little then make them slow. Could even take smoke away with one lmg and AT nade away with the other.
Instead of reducing the effectiveness of rifles across the board, make them highly effective in one or two ways. Make it a strategic decision, not just less of a no-brainer.
unfortunately USF don't have many infantry options, if their one inf squad cant do everything (non-doc) they will simply be left without those options.
Well if you have ever seen Capt. S. Price play USF you will understand why RE's that can equip anything are a problem.