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I found my Celestron C925 to exhibit both coma and significant vignetting if the spacing was too great with the 6.3 reducer. You could try reducing the spacing if you are able and see if that improves the coma (It will probably always have some, mine still does) and if it helps, it will also improve your clearance. Mine worked out as about an F7 reducer where I was happiest with the image.
The remaining coma actually makes sorting out the collimation easy, it is visible at the edges of the field in short exposures and if the tails on stars in the corners were not pointed into the corners, and halfway along the edges pointed straight at the respective edges then it needed a tweak.
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