Argument necromancy, hooray!
I'm definitely in the camp that thinks the Gunner needs increased utility so that having 1 or even 2 is the rule rather than the exception.
Gun mods still need some tweaking to make them a bit more user/noobie friendly, but this might be one of the rare balancing situations where a nerf would be better then a buff. Maybe the Engie is too useful, rather than Gunners being too weak?
What if the Gunner was, like, 'tougher' then other classes. Make other classes unable to use guns that are on fire, and give the gunner reduced penalties for handling damaged weapons. This poorly conceived change that I literally just made up would have three positive effects.
1) Buff the Gunner in a way that wouldn't necessitate major changes to the current ammo system.
2) NERF the Engineer / Pilots trying to use guns, but only a little. They're likely to have a way to extinguish fire and repair a gun quickly, so it's more of a speed bump in their ability to fire a gun, making a true Gunner more responsive and consistent in their offense.
3) Buff fire, which is currently kinda blah.
Essentially the Engie would be forced to stop and fix his gun, while a Gunner could 'take the heat' and continue fighting at peak efficiency until the last moment.