The "stretch" that you refer to is the coma that these scopes display, which is what these f/6.3 reducer/correctors fix, and what the ACF and HD SCT designs correct for in the first instance. As visual instruments, this coma is not optically significant. But for photo it is.
These ACF and HD SCT's as a result cannot make use of the standard f/6.3 reducers because these scopes are already corrected, and attempting to use these correctors will only introduce aberrations!
Instead a different reducer needs to be used with these particular scopes. Again, like I said in my first post in this thread, the corrector/reducer needs to be matched to the scope being used.
Newtonians are no different with the coma corrector used, depending on the focal ratio. Coma correctors are designed for a range of f/ratios, so one optimized for f/4.5 to f/6 won't clean things up so well in an f/4 Newt.
Alex.