I think that Ussop's Kabuto weapon could take out one buster call warship, but not all of them. A fireball is a effective weapon but the farther a fireball travels, the smalled the flame becomes since of air friction.
You're thinking too limitedly. I'm certain Kabuto has more than just fireballs in it's arsenal. For example, I can imagine Kabuto being slightly modified with impact dials that give projectiles an extra 'push' as they're fired, resulting in a projectile that travels much farther and faster than it would have without the push. And a faster traveling projectile has more momentum, and thus more 'explosive' power than a slower traveling projectile. Imagine 5 or 6 impact dials, arranged around Kabuto's ring giving push to a projectile the size of a baseball. Imagine the 'explosive' power created from such a projectile. It would be like attacking with the cannon of an M1A1 Abrams tank x 10 I imagine. The only problem I could perceive with such a weapon would be the 'kickback' produced from it. So it'll probably require that Kabuto first be planted before using impact dials to help fire projectiles.
Perhaps you are overestimating Usopp a bit. The Buster Call is something that is feared by everyone. If it is feared by everyone, then it must be something extremely devastating and it is shown to be able to wipe out an entire island.
If Usopp is able to take them down, that makes him even more terrifying than the Buster Call, and that means he can take down an entire island by himself too! Even if he is strong, that is just way too much.
We're not talking about wiping out entire islands here, we're talking about wiping out ships. And the WG has probably never encountered anything like dials before. The dials, I'm sure, if used in creative/imaginative ways, could prove to be incredibly destructive, far more destructive than any weapon the Gov't may have in their posession.