I think its best to compare the Jackson to the firefly, since they are now pretty similar and they fight the same units. To compare the Jackson to the panther or many axis tanks doesn't give you a great idea of what you are paying for as they are so different.
640 HP, 60 range 220/210 penetration far 260 penetration near for both. They cost 140 and 155 fuel respectively, so Knowing that the numbers should play out that the firefly is slightly better.
If you do the DPS considering they have such similar penetration you get
Jackson : 24.427480916DPS
FireFly :24.2424242424 DPS
They are doing approximately the same amount of damage. The jackson has 1.2 more speed and nearly twice the accelearation as the firefly
FireFly- Speed: 5.3 Accel: 1.6
Jackson-Speed: 6.5 Accel: 3
(im not an expert on how much this helps but the jacksons definitely faster).
To me it seems like you are getting a cheaper firefly that's more mobile, I think reload or penetration should be decreased slightly to compensate for this inconsistency. Mabye USF should have a better TD as it is their one lategame option I think the jackson can be the best and still be balanced, but I think this should be looked into and the numbers should be made more comparable.
As an aside firefly to me seems like its a middle of the road kind of tank this patch, not too good, not too bad.
(I'm ignoring special abilities b/c idk the best way to compare them).