I think the contrast important on DSO too. Then how about long focus Newtonian VS short focus Newtonian on DSO?
Not as apparent if apparent at all. Its a different level of resolution in seeing contrast when viewing skotopic versus photopic as on Jupiter for example. I highly doubt you'd ever see the difference on M51 or M33 - buiuttt - in terms of resolving stellar galactic nuclei it might work in your favor.
Open clusters and globular clusters are another matter. The small CO puts more light into the spurious disk do the rings are fainter which boosts contrast of dimmer stars. Its subtle but when you are at the limits there is no doubt a 20% CO. will show fainter cleaner stars than a 38% CO.
The less you spread out faint light in diffraction patterns the higher the apparent contrast against the night sky.
Contrast is important when viewing DSOs but a larger CO results in a fine scale loss of contrast, not overall contrast and at the low light levels, the eye cannot resolve those fine scale contrasts..