you have to understand that capital ships are more durable, but lower DPS then frigates and MUCH lower then cruisersI don't know what game your playing, but it obviously isn't sins. Cap ships have MUCH higher dps than frigates and even HIGHER dps than cruisers, infact, most cruisers (barring the combat ones) have little to no dps at all. Whereas caps can have upwards of 100 total dps, most frigates won't even reach 20.If your the vasari, keep the bounty up on other players in your system, then, once you've conquered the system, just keep the bounty off you and build 6-7 kostura cannons and shell the pirate base, eventually you will destroy the gauss cannons and severely weaken the pirate ships (this is how i take care of pirates as vasari) not to mention if you can time it right you can jump in while the ships are still disabled.
He means DPS/Supply, yes. For their supply cost, heavy cruisers do more damage and use a better damage type than battleships, at least until the capitals reach a significantly high experience level. In fact, LR frigates statistically have the best DPS/Supply ratios there are, better than heavy cruisers, battleships or even carriers.
Kostura Cannons will never kill anything on their own because they do 10% of target's remaining hp worth of damage. In this anti-pirate respect though, the Kosturas are even better than Novaliths(which won't get rid of the Pirate ships, just their colony). Since they disable ships and turrets, but not strike craft, it would be simple to just go into a pirate base with a fleet of 30-50 Lasurak Transporters supported by a barrage of Kostura fire and wipe the Pirates out, and then siege their base once the ships are gone.