The most problematic balance in jj2 has always been the maximum fastfire. It goes way more frequent than humanly possible to tap fire. Fastfire should mostly just be an convenience thing, but it goes to a big DPS improvement to destroy bosses easily but also slurps your ammo. So in a way you do too much damage, and you actually don't want to be firing that fast at that ammo cost rate. In a way jj2 is a lot about stocking up insane amounts of ammo and then unleashing all of that on a single boss.

I'm more a fan of just capping down the fastfire by 2 levels. It still has this convenience, you can still fire fast, but not cheese incredibly hard.

And maybe add a small 'chance to not consume ammo' with fastfires, although that might be controversial. Not just for Turbo difficulty. Or maybe add it for other difficulties and remove the benefit in Turbo for better ammo mannagement. Many options there.