I disagree with most of what you say and think you are trying to confuse the issue.
Confuse it with the facts? Anything specific you could support this accusation with?
IIn "Telescope Optics" by Rutten and VanVerooj, Fig. 9.3 in the book shows the terrible coma for the all spherical design which anyone could easily see if they looked. The comatic images just 10mm off axis are 10 times the Airy disk diameter for both the curved focal surface and the flat focal surface. They are slightly worse than the spot plots for a f/6 parabolic mirror in Fig. 5.6. So the statement that the all spherical SCT design has coma equivalent to a f/5 Newt. is accurate.
The blur at 10mmm off axis in Fig. 9.3 measures 17mm, and the Airy disc is little over 2.5mm, or about 6.5 times smaller, not 10 times. Since the coma blur is 1.56 times the Airy disc at its "diffraction limited" level of 0.074 wave RMS (0.42 wave p-v),, this implies the p-v error of about 1.75 wave p-v. Just about identical to what raytrace indicates. That makes it 3.5 waves p-v 20mm off axis,
If, as you say, the SCT blur is "slightly worse" than the f/6 paraboloid blur, that doesn't make it comparable to f/5 paraboloid, whose blur at 10mm off axis would've been 11 times the Airy disc. It makes it comparable to f/6 paraboloid, since its blur is also about 6.5 times the Airy disc. Like I said, Rutten and Venroij either erred, or are (more likely) talking about the linear blur size, which cannot be a measure of the aberration magnitude in systems with different Airy disc diameters (i.e. of different focal ratio).
For the spot diagrams of the "optimized" SCT design with an aspherized secondary in Fig. 9.5, if the manufacturer is off just plus or minus 2% on the power of the corrector plate the coma is easily visible at 20 mm off axis as the spot plot is 4 times the Airy disk. If the manufacturer makes a "perfect" optic(good luck with that) the spot plots are much better, but only on the "curved" focal plane. The flat focal plane will of course be worse.
There is nothing "easily visible" about the blur three times the Airy disc diameter at 20mm off axis. It corresponds to coma in f/9.6 paraboloid. Beside, 2% error in the corrector power is a lousy piece of work even by the commercial SCT standards. It equals 1.75 wave corrector profile error, and requires nearly 1/2 wave of spherical aberration correction by aspherizing the secondary.