Hi Paul, I had a chat with Scott Tucker from Starizona. From what I've gathered so far the old Celestron & Meade SCT range have both spherical primaries and secondaries. The corrector plate takes care of coma and spherical abberations. Theorically the further the corrector plate is from the primary the more correction to the coma (without moving the secondary).
So it's fair to assume that if you focus by moving the primary the further away from the corrector plate and add tubes to the back focus you'll correct coma better than having the primary up the baffle tube closer to the corrector. Will try that.
The Meade ACF range introduced a hyperbolic secondary to deal with Coma. Celestron HD range on the other hand have the same optics but deal with coma by adding a doublet inside the baffle tube in the same plane as the primary to correct the field. Other than than the optics are the same. I asked if Celestron would provide the doublet as a retrofit but I hear it is very unlikely.
Having said that I'm looking into
this which is made in Germany by Philip Keller. I believe he makes the Keller correctors in the ASA range and this mob has an SCT corrector (x0.8) wight a pretty good looking spot diagram. So I'm going to try this next and get one shipped.