I've got a network with 6000+ sites. I've built a plugin that displays a table with all sites and the active theme and active plugins for each site, but thought I'd give yours a look. It is a challenge getting all that data in an efficient way due to the way it is stored in the database, so always curious to see if someone else has an efficient way to determine which sites plugins and themes are active on.
I suspected it would time out and fail to load the Installed Plugins page and Installed Themes page since it has to run thousands of queries. Most of the plugins like this that I have found fail to load on my network. That did not happen, however the results returned are inaccurate.
When I view the Installed Themes page, our most popular theme is listed as active on only 6 sites. It is actually active on over 2,200 sites. This is the case for all themes listed on the page. The number of sites listed in the "Active In" column is always less than the total number that actually have the theme active.
It is the same for Installed Plugins page. Some plugins which are active on 200+ sites only list 1 or 2 sites in the "Active In" column.
It is probably only an issue for large networks like mine, but I thought I'd let you know. I'm not sure where the tipping point would be for having too many sites, but just a consideration that this perhaps does not scale very well.