Bounds.intersect is bad, mkay?!

I was trying to check when enemy ships are getting out of screen to destroy objects (optimization). First version

Vector2 screenSize = new Vector3 (Screen.width, Screen.height, Mathf.Abs (Camera.main.transform.position.z));
Bounds screenBounds = new Bounds (new Vector2(0f,0f), screenSize);
Debug.Log ("Ship " + ship.renderer.bounds + "  Screen " + screenBounds);
if (!ship.renderer.bounds.Intersects (screenBounds))
   ship.Destroy ();

So apparently this operation is taking significant time to check, end up replacing with this

  Vector3 screenPos = Camera.main.WorldToScreenPoint( ship.transform.position ); 
  Vector3 screenSize = Camera.main.WorldToScreenPoint( ship.renderer.bounds.size);
  if (screenPos.x + screenSize.x < 0)
   ship.Destroy ();