It's not the Ishtar, it's sentry drones. The vanilla Dominix beats every other vanilla battleship at literally everything by miles. If you're shooting at targets further than about 25km away, it also beats every navy and pirate battleship at everything.
Dominix with bouncers does 371 dps at 71km optimal before damage mods. An Apocalypse with 8x Tachyon II and faction XRay does 349 DPS at this range. Of course, to actually fit 8x Tachyon II to an Apocalypse, you need two fitting mods and the guns alone consume 50 cap per second base, over 60 with a single heat sink. The Dominix's drones also have better tracking, despite the Apocalypse using the guns with the best tracking and having a tracking bonus on top of it. Dominix gets to have better stats with no cap use or fitting requirements. Railguns and artillery are even worse in comparison. The Dominix can also swap between long range sniping and close range DPS just by switching out drones, none of the other gun battleships can just arbitrarily switch from long range guns to short range guns.
Sentry drones absolutely need their damage nerfed by a straight 50% or there's no reason to ever use anything else.