Nope, I have zero meshes in the subsystems and the “turrets” don’t actually rotate.
I just made them as an Actor with no mesh and it doesn’t even rotate.
It has a direction and checks if the enemy is within a certain firing arc. If so, spawns the projectile.
Nothing else, very minimalistic 
The cost is literally due to the fact that these components need to be updated in their position, because the ship is moving and they need to follow her in the movement.
For a few ships this is very low cost, but for many ships it becomes too much for the CPU 