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There is usually some penalty to shortening the light cone...it's already evident with the f/6.3 reducer, as there isn't as much back focus as at regular f/10, to the point where some eyepieces (mostly 2" ones) won't come to focus.
Then you reach a point where the baffle tube cuts into the light cone and causes vignetting.
In practice, I've found that the best way to maximise the FOV of my C8 is to use a 2" long focal length eyepiece, such as a 40mm Paragon, which works really well for about 1.4 degrees. Any objects that don't fit in that are probably better viewed in a very short focal length refractor IMO.
Meade currently produces a native f/8 SCT and have promised a reducer taking it down to f/5, but it has yet to go on sale. The down side of the shorter native focal ratio is the secondary obstruction is noticeably bigger than the f/10 models. There's much debate about how this would affect the image but very few posts about the real experience with the scopes themselves...so far, they're pretty recent additions to the range.
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