Do you find that, once trained [and pinned], extremely qualified people do a noticeably better job once they're in? Or does the lack of actual time doing the job (if they're not mobilized) act as a ceiling for how well they can perform?
Oof, great question. Absolutely depends on the person. The qualification (in theory) should establish that baseline knowledge for doing the Navy job but the Reserve is also notorious for pencil-whipping qualifications and boards.
In general, I would say the DCOs tend to be more mature and can leverage that outside professional experience when it comes to managing sailors or dealing with the day to day minutiae of Navy life. I think what is often overlooked here is that officers are not on the line doing much of the work regardless of your designator. That falls on your enlisted sailors and your job is more of a mid-level manager than anything else (with exception, of course).
You also see DCOs who think because they had X job in the private sector, they somehow are better at their Navy job than their peers (happens a ton in the DC area). Those people tend to not stay in the Reserve very long.
That being said, your active component peers will just be much more experienced in doing the actual job.