Outcasts destroyer turrets have fighter turret model, so that they would fit the previous Outcast destroyer model (= Kushan Homeworld II Heavy cruiser).
They could probably use a bit less energy yes.
Kusari destroyer/defense turrets are from vanilla to fit the different positions on the model.
Igiss says: Martin, you give them a finger, they bite off your arm.
By the way, i just wonder, why there is only 2 factions with cruiser destroyer defense turrets? Why not to add them to other factions too? Since turning speed of primaries is not enough for bombers and fighters.