I've got vignetting with a 6.3 reducer and a Celestron C8 too, it's correctable with flats when I shoot in DX mode (Nikon's term for cropping at centre APS-C sized part of the sensor). When I shoot at the full 35mm I get completely uncorrectable vignetting.
Your results looks quite good. I'm experimenting at the moment with the spacing between the camera and the reducer. The spacing seems to make quite a difference in coma but I haven't quantified it yet.
Would you be able to share what your current film plane to reducer distance is? I'm not sure if the NEX has a film plane indicator on the body but if it doesn't can you provide the distance from the corrector to the mount on the camera? Currently the coma on my images with the reducer is quite bad:
http://www.flickr.com/photos/1009024...in/photostream