Apologies for being late in response. I am not sure how you would define a 'superuser' necessarily, unless it is simply on the number of artworks loaded (which I don't think is a good idea). Since I joined I have been passionate about getting a) information correct and b) ensuring that as much info and facts are loaded into the system as is known. I do spend some time adding facts to existing pages. I find that a lot of information is missing and that is a pity, if we are to show that this is a responsible website. One of my irking complaints is that some users do not put in the size of works (after all, you provide both cms and ins for this facility). All museum websites give dimensions, so it is really no excuse! To understand a work of art properly, one needs to know its overall size. This is perhaps not so important with copyright images as these are small but one can still include the size.
Another irritating complaint I have is that there are a great number of works that go into the 'unknown' category. But in many cases, the existence of these is known and some simple research will most probably uncover the details. I spend some time also in filling in either the 'whereabouts unknown' entries and also quite a few 'private' entries. It seems that if an uploader doesn't know where the artwork is they place it under 'private'! This is at least better than 'unknown'.
Anyway - enough of my moans!
A superuser could also have the ability to delete entries in the dropdown list of, for example, museums. As Josselin pointed out, there are a number of museum entries that have become redundant. I find that the title disappears from the main index but is extant in the dropdown list. This is confusing for uploaders who will see a listing and put the work in question under that title which is in fact non-extant. I have recently removed 'National Museums of Liverpool' and moved the five artworks to their respective museums (Walker Art Gallery and Lady Lever): These already existed with a large number of works so it was a duplication. But the dropdown list will still show 'National Museums of Liverpool' and this will be confusing for uploaders.
By allowing a 'superuser' to have this ability, will also free you from having to spend your precious time.
Additons and changes to an artpage should also be recorded (as you say for any mistakes or malicious use).