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I've not seen any specs for the f/6.3 reducer for the XLT scopes, but it's the least expensive option. The clear aperture is about the same as the baffle diameter on the C8. I'd be really surprised to learn it could illuminate a full frame chip, so you'd want to check what the vignetting situation is, as the light drop off will be more noticeable in long exposures than it is visually.
The C9.25 is slightly flatter than the other scopes in the range due to having slower mirrors, but it's not much.
The flattener in the Edge HD scopes is located somewhere in the baffle tube, it depends on the model and is shown schematically in the white paper. It doesn't change the focal ratio, but reduces coma and flattens the field somewhat. The reducers for the Edge HD scopes are just that...reducers.
Another option to consider is the Ritchey Chretien type scopes, as they are also long focal length scopes, and they're popular with a number of folk in here.
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