Actually, the assault gunboat is the middle equivalent between the YW and the BW.
Many people tend to confusse ship equivalencis as they just compare speed and weaponry. This wa smade this way not to make Imperial Navy and Rebel Armada just mirrors from each other.
For instance, the equivalent of an X-Wing is not a Tie Fighter, but a Tie Interceptor. Not only it shares its weapon and armamaent distribution, speed is allmost equivalent (remember standard Ties don't have shields, so to compensate for that, the interceptor was given 10 extra MGLT speed). The actual equivalent of the Tie Fighter is the Z-95 headhunter, as it iis slower tan an XW and shares the attack power of the standard Imperial navy fighter.
Next you have the YW, whose equivalent in terms of speed and raw firepower is the Tie Bomber. Allbeit the lack of shields, this ship isn't as fast as a YW but performs its role in a similar way: wipe out enemy ships. Though YW also have Ion cannons to perfomr capture operations, in Tie Fighter the BW is the prime hunter for the rebels when it comes to capture enemy ships, much like the Assault Gunboat is the standard choice for Imperial Navy capture operations. Hence why the Gunboat is a middle term between YW and BW, but in terms of speed and manuverability its closer to the BW.
AW's equivalent, allthough exagerated is the Tie Advanced. I know it seems strange given the T/A superior speed and weaponry, but both ships have same manuverability, and the A-W has slightly stronger shields.
The two ships in the imperial navy that don't have equivalent are the Tie Defender and the Missile Boat; the T/D is overpowered, period. In fact, many clans banned it's use on multiplayer matches due to the fact that it was the fastest ship in the game (even with full power redirected to shields and lasers, the ship is as fast as an XW with full laser rechargge), strong shields (only surpassed by YW and Gunboat ones) and superior weaponry (four laser canons and 2 ion cannons, plus great capacity dual warhead launchers). And the missile boat, allthough not a ship to be played competitively, it can outrun ANY SHIP (even T/D), not to mention that it can rely on 80 advanced concussion missiles to take out ships.