to cut away a little bit on the wife angle stuff
i think that this M67 filter adapter from inon is very interesting if you add the 67mm variable filter that they have
http://www.inon.jp/products/gopro/hero/sdfrontmaskhero9.html
i did it a few times, although for sure its not as wide, but it helps with the white balance.
i wish now, that they can combine these 2 things that they have.. put a smaller variable filter before the wide angle lens!!!