I was looking for advise on the weight comparison of the PolarPro ND filters (inc the ND PL filters) compared to the DJI stock ND filters. I am concerned that the weight difference between the two brands may be as much as 3g heavier in the PP brand. Therefore is there any concern for the extra load imposed on the camera gimbal?  

Good question, I have the same concern. Have seen pictures of the P4P camera with a filter on it (don't know which brand, though) and it was facing completely down, with the drone turned off.