Well, a few arguments here. But mainly, I don't agree the Heavy Carronade should be touched at all.
Changing the rate of fire would not help much because buffed Heavy Clip will still take down the Balloon. Reload time would not do much to help the Balloon either, it will still go down on the first clip, only its overall DPS will be reduced. What you might be asking for is a severe damage nerf, which will prevent the Carronade from popping balloons in one clip even when buffed.
Goldfish is the only ship with enough mobility to bring the Heavy Carronade close enough. A meta-Galleon uses it defensively, and with the acceleration buff, it can go on a Spire but we have yet to see much of it. The Goldfish can be countered - Pyramidion is a classic way to beat it because of it's protected Balloon and superior firepower against the Goldfish.
The Heavy Carronade has 425m range, and the challenge is being able to get the Goldfish close enough without getting disabled or killed. You also need time to ground out and kill the enemy while the other enemy ship(s) bombard you. In my opinion, the fact that Balloons require high maintenance and there are altitude battles makes GOIO interesting.