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I have noticed an increase in SNR when I used a reducer on my refractor as opposed to imaging at the native focal length (used the same camera in both scenarios).
A 15-minute exposure at f/4.5 gave a slightly better SNR for extended objects than a 20-minute exposure at f/6 (same scope, same camera). I tested that for multiple filters and for multiple exposures.
Faster f-ratio does speed up imaging of the extended objects (nebulae, galaxies) if the same camera is being used and same scope, but for the stars it is less of a difference, since these are approximately point sources therefore aperture dictates the signal detected from the stars, not the f-ratio.
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