Well firstly, I'm not really sure its possible to make X unit fire slower if there is another one near it. It might be doable by creating some sort of invisible auras, but it sounds way complicated to begin with.
And secondly, ROF isnt the main thing making Jackson so good. Its accuracy on the move, speed, turret rotation, turret iself, range. Pretty much any aspect of Jackson (aside from armor) is making Jackson so powerfull compared to other TDs. I mean, if you litteraly wont make Jackson ROF x2 slower (so PIV will shoot 2 times, while jackson will shot 1) it wont change anything.
But even with such changes, ppl will just kite with jackson. It will make it harded to micro, but ultimatly wont change anything for a good players, just make it really frustrating to use for mediocre players.
One can make TDs have higher TTK vs medium without affecting the TTK vs super heavies.
A) Lowering far accuracy will effect the shot vs medium more than it will effect shot vs super heavies.
B) Increasing damage but lowering ROF can be used to keep the number of shot the same vs meduim and the TTK about the same vs Super heavies.