No performance boost on culling when using "CameraFacingBillboard"-script

I set up some occlusion culling + default frustum culling. In a test area I have 3000 billboards/sprites. Once I attach the "CameraFacingBillboard"-script (in C#) the fps-count is still very low (around 5 fps) when I pan the view although all the billboards got culled and none is visible.

Therefore it appears they are still turning towards the camera even though they got culled. Is there a way to disable the script once they are culled? Or any other technique to achieve better results?

Here is the script:

using UnityEngine;

using System.Collections;

public class CameraFacingBillboard : MonoBehaviour

{ public Camera m_Camera;

 void Update()

 {

     transform.LookAt(transform.position + m_Camera.transform.rotation * Vector3.back,

         m_Camera.transform.rotation * Vector3.up);

 }

}

Thx