The main purpose of a Circular polarizer is to reduce glare/reflection that appear on the windshield. If you are not bothered by the glare/reflections then there is no need to add a filter to your dashcam. NOTICE, how I said reduce the glare not eliminate.
The CPL filter only works at right angles (90 degree angles) towards the sun. Chances are you wont be at that angle all the time and so the reduction of the glare is less.
CPL also gives you more vibrant colors. I posted this fame from a video earlier.
http://i.imgur.com/HFhCnPg.jpg
So if you can't justify your time and $3 for a filter to achieve those results then don't bother with this mod.
When I first got the camera, I was really disappointed with the quality of the videos I was getting. I actually paid close to $350 for this camera and when I saw the videos, they were about the same as a keying camera that I bought for $34 from ebay. After seeing the quality, I went searching online to see if anyone else was getting ****ty videos. I found a Russian forum and after pages and pages of translated Russian and through a lot of messages I was able to get the custom firmware. Changing the bit-rate of the videos was a lot of improvement and with the latest firmware the quality has improved greatly from when I first saw the videos.
Now I don't feel so bad about spending that much money on this camera. But if it wasn't for the custom firmware I'd have returned this long time ago.
If you're happy with the price to performance ratio of the camera out of the box, that's good. But I personally feel that adding the polarizer has improved the quality of the videos even more. I have Cannon 60D and 5D Mark ii, I don't go comparing the videos of those DSLRs to this dashcam. I know DR500GW is not a movie camera and I'm pretty sure everyone who buys this dashcam knows that. But improving the video quality doesn't mean that people intend to use it as a dedicated video camera.
Also An overlay wont work because you need to have adjustment in the filter. A fixed polarizer wont work or will only work at the angel it is fixed at.