I don't really see where or how could they be buffed.
The only unit I see that could be buffed is greyhound, which could(should) get smoke as M20.
Tanks? You can't really make shermans stronger and there is nothing really that could be done to make jacksons/wolverines/scotts stronger without breaking them. Well, maybe except wolverine.
I think it's a mistake to leave overpowered abilities in place because of underpowered units using them. I believe canister shot should be nerfed but M8 main gun damage increased, for examaple. Here are some other examples that I don't think are game-breaking at all.
Greyhound needs more utility through smoke, mines, or a top gunner.
Stuart and M8A1 could use a little more health so that they can take 3 shots from standard AT weapons and 2 shots from the pak 43 and jagdtiger. Stuart needs a cost reduction to make any sense to purchase at all over the M8A1.
More armor on shermans would be nice since currently it makes almost zero difference if you charge in reverse. Almost everything will penetrate a sherman's frontal armor except max range puma and p4 shots.
M10s need more health and accuracy.
Jacksons need better acceleration and a buff to AP rounds so they are more reliable.
M8A1 needs a larger AOE, less scatter, more responsive barrage, slower rate of fire, and more aoe fall-off so that it does consistent damage to squads rather than miss miss miss miss squad wipe.
I would advocate for all of these changes regardless of changes to repair critical. I think all of these changes are necessary to make these units balanced, and likewise I think the changes to repair critical are necessary to make that ability balanced.