The episode does illustrate an interesting option: a small suicide ship can outpower and outmaneuver a much larger ship.
The Orion attackers are described as fast and maneuverable and sensorproof and possessing moderate firepower. But as far as dimensions go, the terminology chosen is "the size of a scout ship". Hardly a case for fightercraft here.
fighters being used against other small craft is really the only way to justify that sort of thing
Either that, or the gunship thing you describe... In military hardware design and construction, "superior" tends to be better than "inferior" - so whatever fighters we see, we should be able to deduce that their intended targets are smaller, weaker and/or possessing less range and mobility!
Interestingly, "Nor a Battle" seems to show that air superiority can be challenged by mere ground troops: the "hopper" of the Starfleet platoon was shot down, Bashir never dared take the runabout in the air, and there were no strafing runs to halt Klingon charges or break ground impasses. So it appears that the small craft of the Trek universe aren't "overkill" designs, but barely adequate for their minimum tasks, just like ITRW. Which in turn would seem to suggest that it's far from trivial to give good protection to small craft.