I have asked the SA this question before, BMW says to leave the embedded stones alone and don't try to straighten the fins yourself. I agree. Some enthusiast in Japan put a layer of thin mesh behind the grill but it may affect airflow. Similar to racemesh but finer.
There are some tools which can be used (I strongly discourage you to try unless you are experienced)
http://www.streetrodderweb.com/tech/0908sr_graham_tool_company/photo_02.html