Also since I’ve recently been down this path. The 6.3 reducer really needs a smallish sensor to play nice with. My asi224 with a 6mm sensor is fine at 6.3 but a large format like my asi294 at 23mm must be scaled down in order to combat the coma and effects produced. I scale the 294 sensor size down in order to make them meet up optically as close as possible. This is for dso imaging btw
In several places on the web including Celestron (at some point), the image circle of a C-8 and up with the f/6.3 reducer is given at 24 or 25 mm. For example here is what Agena Astro says about reducers. It is worth a read because it covers aspects that we sometimes overlook.
4.1 Field of View and Image Circle
For imagers, the main purpose of a focal reducer is to increase the brightness of the image at the focal plane. But while the image gets brighter, the size of the image circle gets proportionately smaller. For example, an 8" SCT without a focal reducer has an illuminated field of 38mm at 50% fall-off. Add a 0.63x reducer, and the brightness of extended objects increases by (1/0.63)2 = 2.5. But the diameter of the image circle decreases by a factor of 0.63 to about 24mm. Such an image circle is still large enough to encompass the relatively large sensor of many deep-sky astronomy cameras. If a stronger level of focal reduction is used, say 0.5x, then the image circle may be too small to fill the sensor of larger cameras.
How about for visual observers? Using this same example of an 8" SCT and a 0.63x reducer, a visual observer can also enjoy brighter images and a wider field of view. But the smaller image circle means there is a limit to the field stop of an eyepiece that can achieve an unvignetted image. An image of about 24mm across, approximately, allows an observer to use a 1.25" eyepiece with a maximal field stop. That includes, for example, a 1.25" eyepiece with an apparent field of view of 68° and a focal length of 24mm (eg. a Tele Vue Panoptic), or a Plossl eyepiece with an apparent field of view of 50° and a focal length of 32mm. With this telescope and this focal reducer, it does not help to move to a 2" eyepiece and a 2" diagonal as the visual view will be akin to looking through a porthole within the larger apparent field of view of the eyepiece.
Another factor to consider: focal reducers also increase the angle at which light approaches the focal plane. This may be a problem if the focuser tube or the diagonal (for visual observing) is too narrow to accept light at this larger angle. As a result, the smaller tube may cut into the light cone and effectively reduce the working aperture of the telescope.
Visual observers are more likely to overlook a degree a vignetting that would be instantly obvious in an image, but imagers can use flats and or correct in post.