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My understanding and experience agree with what Colin has written about F-ratios and photographic speed. Large aperture will get you more stars and they will be brighter largely irrespective of the F-ratio, while faster F-ratio will give your subs a greater SNR for extended objects. (This is actually good news when using pixels with shallow wells on a fast small aperture telescope, because stars do not saturate very quickly while we get good signal on extended objects)
For example: 15-minute subs at f/4.5 still had a higher SNR than 20-minute subs at f/6 (same aperture and same camera, same mount etc) - verified on a number of nights when I was going for over 200 hours on Helix.
With a short refractor - personally I would not want to go faster than f/4.5. Focus shift becomes significant and precise orthogonality to ensure a well-corrected field is also becoming a challenge when using small pixels. I also agree that the corrected flat coma-free field shrinks rapidly when we attach stronger reducers.
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