Can't speak to the RC8, but I can tell you that there is little vignetting when using a APS size chip on a Vixen VC200L, even with the 0.71x reducer. A flat (unstretched) from last night is attached. The VC200L includes a triplet flat field corrector as standard, so even without the reducer the field is fully corrected out to a 44mm image circle (ie a "full frame" DSLR).
A VC200L is a lot more expensive than a RC8 though so that is obviously a consideration. The focuser is from all accounts a zillion times better than the GSO though (not hard to believe if my GSO newts are anything to go by), so no need to upgrade there.
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