I would like to add to this thread, that I was using a DJI ND16 filter for P3 on my P4, because it was the only one I could find. So I noticed little to no difference at all regarding the photo and video quality, needless to say I had my doubts about third party filters and the need for them.
Living in a country with no reliable postal service and unable to find any Polar Pro products, I asked my friend who was coming back from Canada to get me the Polar Pro Shutter series (ND 16-32-64) and the gimbal lock from a shop in his area.
Both of them cost me around 145 US Dollars including taxes.... So I was really afraid that, having slightly overpaid for them, my doubts will materialize and I won't notice any improvements in photo and video quality....
Yesterday was a hot bright and sunny day, so without getting my hopes up, I decided to go do some tests..... Boy was I wrong!!! I cannot believe the difference those filters made! Seems that AR coating is really doing one heck of a job! Photos were crisp, sharp, no flaring and a lot better than the DJI filter, not to mention the video quality which was awesome!
So in all, I'm very satisfied and don't regret slightly overpaying for the filter and gimbal lock (comparing to US price)... I wish they made an ND4 and ND8 of the shutter series!
I still have to fabricate a filter case for them now

Any suggestions?