I not sure either, but I'm using an astronomik front filter and my custom white balance shows vignetting similar to what your seeing. The raw file looks like it has a spot in the middle when converted to RGB in maxim. I don't get any distortion in images when using the CWB but I used to without it
I have attached my CWB file but this one looks a lot smoother out of PS than it does on screen in maxim. What do your normal daytime shots look like?