Pete
The darker the better confirms the factory input.
My camera is certainly dark.
I think that when it's darker the shutter speed is longer and therefore tends to blur the prop more.

My camera came with a mount for a headset.
I am going to try that next.

In the meantime here is a video with several turns into and away from the sun.
You can see a definite difference in the prop blur depending on the light level.
If you run full screen it shows the effect better.
http://contour.com/stories/cc-landing-ump-12-28-2011--2

I do like how the aircraft audio records on the video.
Bill