How much more work does on average a moderator have on this StackExchange, compared to a higher ranked user? Is it just having to have some last word on some close/flag/ban disputes, or is there more to it?
I don't know if you're going to get a usable answer to this. Even calculating an "average" workload has such a wide variance as to be meaningless. It depends wildly on the moderator, the scope of the site, the involvement of the community, where the site is in it's development, the alignment of the planets, what's on TV tonight, etc.
Moderators are there to be human exception handlers. Diamond moderators participate as volunteers when and where they see fit. It's not a "job," either full-time or part time. So the amount of extra work for a moderator varies from "none at all" to "I can't handle the load; need help."
Sorry, that's the best I've got.
Speaking for myself, the load has been pretty low, near zero. I would say my normal participation on the site is at least ten times the amount of stuff I do with my moderator hat on. I think I've taken, maybe, ten moderator actions and spent maybe two hours actively coordinating with the other moderators.
There just haven't been that many issues. 99% of a stack exchange site is the site running itself. Half the things we do would have done themselves in a few more hours had a moderator not intervened.
I know certain other moderators (who will not be mentioned, but are getting married this weekend) have done more, doing things like actively promoting this site on Twitter and Facebook.