Sherman has great pen on its AP shells iirc. Don't recall the numbers but isn't it 140 close and something like 120 far? That's pretty great and again, I'm going off memory here but the AP shells also have an AOE of 2 (VS the regular 2.5 of multi purpose shells) so you are not trading so much AI for pretty solid AT. It might be interesting to see what knocking that AOE down a bit and seeing what elevated pen would look like (say give it TD AOE but like +20 pen on all ranges) then the Sherman could be the medium killer OR infantry killer but falls off VS combined arms approaches.
Sherman problems is its survivability, not its DPS vs various targets. Knowing all infantry vs you can snare you make it irrelevant in almost all situations and I have been a fervent user of Sherman "spam" strat in 2vs2 till rank 50/30.
There is nothing insentive to build a Sherman over a Jackson. Over-performance of Jackson is also due to you have to build it yes or yes, even more if you went Lieutenant first. Actually I'm using a lot Armor company so I can diversify my late game build. A M10 is enough vs medium and let me get faster Scott, then I can build a Jackson if needed.
This is something I have say since long but Jackson can never be balanced since it has to fill two distinct roles and with other AT source in support. In comparaison UKF can always field Atgun or a AEC to support its Firefly, or simply to build mines to protect its flank.