No, you literally can not do anything to AC2 and AC5 to improve them, without me becoming horribly OP with them again.
There used to be a time, when i alone could, in a quad AC5 Partyvan, both shut down the entire relevant airspace on a map AND also shear off the heads of Atlas Mechs.
Because back then, the AC5 had a much lower reload time, much higher projectile speed and actually used the missle code as far as i know.
Which means it created a more or less constant stream of explosions with screenshake on target.
And it did not have as much projectile drop either, in addition to the increased actual projectile speed.
THAT was when the AC2 and AC5 both were still usefull weapons. Nowadays, no, they are not.
And if you think about it, you will understand why:
A QuadAC5 setup means you have an AC20 that has less bullet drop, faster bullet speed, MUCH faster firing rate.
Yes, that is horribly OP. And to make that not be the case, they had back then already, had to change ACs to the sorry state they are in right now.
And surprise surprise, shortly after that change suddenly came a time of Air-Asset-Dominance, because both the Partyvan AND the Huey suddenly could not provide adequate AA anymore.