I'm not so sure a change in heavy ammo would necessarily harm the other guns it's popular on besides the gat/flak. At least after some consideration of adding say a 20% damage decrease instead of (or in addition to) the 25% reduced muzzle velocity.
As already noted, a Hwacha with heavy deals almost no component killing damage. Nerfing it more would just make it a CQC weapon like the carronade (which would suffer from it too since you can't aim down with it and you would be loosing the additional help you might have gotten by concentrating the pellets much more on the aim). That wouldn't be so bad, but there still would be the same proportion between those guns too. It would help keep the hull armor up longer, sure. But since you won't able to fire back as efficient as before...where would it lead to? Maybe guns that dont need heavy clip would be more common then, like a heavy flak fish. Since almost everyone will still be using it there's just nothing gained. The fight won't be getting any closer except people would start using greased - but your engineer has to think about using greased or heavy for the ranges, since he may only take one. That may add more tactics into this loadout but unexperienced players would have a really hard time. And since other guns would be nerfed too (Hwacha being only CQC with greased/burst/charged like the carronade) no one would use heavy anymore. You could just take it out of the game. The light flak would be a CQC weapon too. Which weapon would you use with the merc now? Almost completely useless too (except for a Spire).
To sum it up: It would lead to many changes, helping the strategic parts of the game but probably killing some fun (especially for beginners), making some guns and ammo types close to useless and destroying even more balance of the game.
It may have many ups but also many downs. I'm right now neither saying that I think it's good nor that it's bad.
The manticore and carronades both benefit far beyond the gatling and flak from the 80% recoil reduction, which for those weapons makes up for that damage loss.
Look above.
Besides increasing engage time and offering risk/reward for firing non heavy flak, this also promotes the use of gunners. Gunners who can bring damage enhancing ammunition for these weapons to switch to once their opponent is close enough to engage with full effect. Also, I could see higher use of buff engineers to offset the damage reduction rather than make it all the more deadly (which I admit, I'm a huge fan of doing... 30-50 round armor peel is devastating at max range with a good flak gunner waiting).
Look above.
That arc is the main reason I see it not used much. However it also takes longer to achieve a strip compared to a good shot on the gat due to the need to reload. Also a single miss is devastating to it's efficiency.
If you stay far away with a flak as second gun there is nothing wrong with that arc. I'd like to see those two in combination, and the gat and mortar in combination. That would be pretty awesome. They would just have to decrease the effective range of the gatling, which would lead to the increased engage time you mentioned before but without the problems of having other guns suffering from it too. Also you could change the firing speed of the gat, which then might lead to the also already mentioned specialization of either greased or heavy. Said that I'd prefer them to nerf the gatling instead of a whole ammo type in favor of the balance.
Changing ammo wouldn't probably lead to anything since other guns would be getting worse too. Just look at the Hwacha (as already noticed).
Anyway, for me, the arc of the gatling isn't the main problem, the stupid range it has for an automatic weapon is and how it doesn't matter how close you are to the enemy for it to deal maximum damage.
Of course, Heavy clip is the main reason for the gatling's extreme range efficiency, without it, the gatling spread is quite large and it can use Heavy rounds with absolutely no downside, maybe it IS Heavy clip that is the problem after all though, since it is extremely useful on way too many guns.
The arc might be a problem too since you may keep on firing the gatling on a pyra for example even if the enemy is almost at 180° on one of your sides. I agree though that the range is a real problem.
Heavy clip is used in many guns - but it's pretty useless in others like heavy flak, lumberjack, merc, mortar... So it's pretty even. Nerfing heavy clip would lead to a dominance of these weapons which, after all, would lead to another balance issue.