Gunboats / gunships are fairly balanced. On average, 2 - 3 should be able to take down a single cruiser / destroyer. In addition to swatting down bombers, that's what they're suppose to do.
That said, I really do think that cruisers need love. I'd suggest a primary turret refire rate of at least 3.03.
@Montezuma: I'm sure you underestimate your piloting skills. It must be said though, that the Corsair gb is a monster and the BHG BC has an "interesting" set of firing arcs. By "interesting" I mean "sucky". It shines in the anti-cruiser department but gets clobbered by anything smaller if the pilot isn't careful.
@Ivan: The Kusari dessie is sort of an anomaly. If it were used more widely, it'd be nerfed 3 ways to Sunday.