Problem with Turing the E8 into a Panther is you will STILL have a Jackson along side it. So then you have a durable meatshields that can punch at heavy cats AND a 60 range powerhouse that can outrage, brawl and chase. It's no contest between being in the shit or outside of it shooting in with no risk of return fire. That's the problem with the E8. It's got no purpose because the Jackson makes it obsolete.
There is no scenario where USF Sherman variants work while the Jackson is the perfect AT machine in every single scenario.
The only way the E8 could find a nich is as a command vehicle with extra abilities that make its unneeded role irrelevant. Sorta like the command panzer where the tank itself is next to useless but the abilities it brings have value.
You seriously undervalue AT/AI tanks. Jackson is a powerhouse in 1v1 and 3v3+ OPEN maps. On closed up maps like Hamburger port or Angrymundy, Essen Turdworks, Ettelbruck Turdstation it's value goes down due to the lane-y nature and really no benefit of having a turret.
You could easily use a bit beefier tank that has decent AT but also has AI (something that jackson has absolutely 0 of).
Furthermore, in 1v1s it's potential would be heightened since you won't encounter more than one panther and in case of P4s, it will win 1v1 (only thing that can change that is the support you/opponent is using).
Lastly, a tank like E8 would be the USF version of "Hail Mary attempt" in lategame teamgames. Currently Panthers, T34-85s and Comets can be used that way. Panthers probably the best because of the pintle MG (which will target infantry 360, unlike turret MGs which target infantry at turret vector or hull MGs -- tank vector).
The only thing that I would further buff E8 with is the armour and increase the price by 5 fuel (230 armour).
E8 could be a tank that will lose to a Panther 1v1 but has smoke canisters and better AI (and with the commander upgrade on it, can focus AT or AI) and there is always a fact that there are other units on the field. A panther might zone out the E8 but your AT gun/ pak howis can zone out the panther/pak40. Still a lot more micro than the OST, which relies on more expensive but tougher tanks, but with good execution you can have a tank that has value in 1v1 and 3v3+.
2x E8s would be a good hunting squadron compared to Jackson + Sherman HE that would not be susceptible to dives.