In the thread "XLT eyepiece performance test" it was described by a poster that by refocusing he could detect field curvature (presumably of the image of the objective, assuming no significant eyepiece field curvature) in the C8 SCT at about 50% of the field radius, and higher. That was part of the argument that the f/6.3 SCT reducer/corrector "significantly flattens the field", since with it, according to the poster, significantly less refocusing was needed. I intended to post this in the mentioned thread, but since it was locked, I have to do it this way.
Raytrace simulation below shows coma in the widest visible field in the C8 (the poster wouldn't specify which eyepiece he was using, so we'll assume the widest field, when the effect of field curvature is the greatest) on both, flat and best curved field (R=-245mm). Assuming 30mm f.l. eyepiece - a well corrected Nagler-like with ~80deg AFOV - so the Airy disc angular radius (4.6/P, P=exit pupil diameter in mm) is 1.5 arcminutes, the sagittal coma - lower 1/3 of the comatic image seen in the eyepiece with sufficiently bright stars - is about three times larger (nearly 5 arcmin) at 50% radius, and six times larger at the field edge. In either case it is large enough to be seen as a blur, so no significant refocusing is possible. There is some difference in the blur shape, but at the actual scale it is indistinct, more so considering that eye cannot see the flat field, since being near-instantly accommodating to near its limit of accommodation (in other words, the blur before and after refocusing - if it would be possible at all - would be even more similar).
Accommodation required for the 50% field radius is 0.14mm, which with 30mm f.l. translates to less than 0.5 diopters, i.e. near-instant accommodation, making any refocusing impossible. Even at the edge, needed accommodation is less than 2 diopters, to which most eyes will readily adjust (average eye with the smallest distance of distinct vision of about 25cm has 4 diopters accommodation power). Evidently, field curvature in the C8 cannot be detected by refocusing.
In the case of observer with much lower than average accommodation power, effective refocusing would be impossible simply because of the overall similarity of the flat vs. curved field blur. So even the fact that f/6.3 reducer, by making the image field correspondingly smaller, significantly reduces apparent field curvature (if it remains nominally unchanged), wouldn't change nothing in the fact that coma prevents effective refocusing w/o reducer. Since reducer does correct coma, or most of it, its field should look cleaner. But that does not equate "less curved". Raytrace shows that reducer of this configuration actually makes field curvature somewhat worse, but the combined effect of smaller field and corrected coma makes the visual field look more "in focus" or, as people often mistakenly describe, "flatter".