Just to make note relating to GoPro manufacturing tolerances that may effect some HD2 camera and housing combinations above water even with SRP provided filters, causing a very slight vignette in HD2 16:9 wide fov video modes.
There is a fairly loose manufacturing tolerance by GoPro so the lens location and how the camera is seated inside the housing can cause this with some cameras and housing combinations. It is very slight and I edited and uploaded many videos not noticing some of my shots were affected by this until I looked back at the footage much later after being made aware of this.
I have 4 housings with a Blurfix adapter 2 HD1's and 2HD2's, this effects one of my housing's and one of my HD2 cams but its so slight its barely noticeable. Underwater all HD2's and housings Ive ever seen with the Blurfix are vignette free in these modes but maybe 5-10% (pure guess) may be effected above water. Even the GoPro dive housing is bound to have some cameras that will have a slight vignette above water due to these slack tolerances, underwater the magnification will avoid the filter frame from being in view so this is only an issues above water at times.
Here is a pic of the vignette level on my "bad HD2" 720P60 using the clear filter by SRP, this is the worst vignette Ive ever got above water using a blurfix and SRP filter and its still not terrible but noticeable. This could easily be cropped out or adjusting how the camera sits in this housing could also eliminate that issue. I think its best to point out for anyone looking at getting the Blurfix that its possible this may occur but unluckily its just pot luck in the GoPro roulette.