After reading all that... and being a 'newbie' to Disco,
Not having flown either a gunboat or a destroyer/cruiser I can't attest to how they handle.
But I can say that I've fought both in my itty bitty Roc. A lone bomber can stand a pretty good chance against a lot of gunboats, including the KNF one.
Oh and a pair of bombers or a trio of fighters, as others have said, will destroy any gunboat, often even if they have fighter support.
Gunboats are fine where they are. (Except maybe the Dragon Hachiman, which I've seen torn to shreds multiple times. :laugh:)
As for Destroyers/Cruisers Pilot skill is primary factor for/against them.