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The magnification is evident as this only works as an f/6.3 reducer when setup with the correct focal length of the reducer itself. I think this was something like 89mm. I.e. it's an f/6.3 reducer if the distance between the sensor and the reducer element is 89mm, otherwise the number changes.
As the focal length in your setup changes you change the magnification, and naturally as a result the vignetting too. Only problem is that coma correction is only deigned to work at one focal length.
From what I've been told with the focal reducer you should be targeting minimum coma and not minimum vignetting. The vignetting will sort itself out with flats providing you're not actually obscuring the edge of the shot with the sensor (like many full frame cameras do on an SCT). This is one of the reason I always shot my Nikon D800 in cropped mode as any advantage of a 35mm sensor was lost due to the added vignetting and poor edge sharpness.
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