Zane, Your post got me to thinking (scary). I have never messed with filters etc. so I started googling them for the HD gopro. I stumbled on this site and gave the guy a call.
http://www.eyeofmine.com/gopro/hd/prof/index.htmlHe has alot of cool mounts that we could use but of more importance, the ND filter and gave me the idea for an external monitor that I can mount in the cockpit to make sure the subject plane etc. is in the shot. The down side to making this phone call is that I just lightened my wallet a bit buying a couple mounts and the monitor.
He did caution (or schooled) me on the proper use of the polarizing filter. He said to make sure you hold the filter up to your eye and look at the sky etc, and rotate the filter to make sure you get the effect you want ie. if it is a real sunny blue sky day, you can roll the fllter around and it will make the sky purple due to the polarization. He said once you get the filter positioned how you want the effect, then carefully drop it in the housing and then put the camera in place and your go to go with the shooting.
I hope to get some good weather when I get home next month and should be able to get some good comparison video with and without the filter.