It would be useful to be able to return the size of a diff and/or cache status before actually preloading it via the browser. For example:
This generates 168k gzipped, but in actuality it is 5.5 megs plaintext.
With &rvdiffinfo it would be nice, if the diff is already cached, to show the size (and possibly titles, cache key, other diff info) without necessarily fetching it.
I am ambivalent on whether it would be a good idea to return the info if the diff is not cached, as that would involve generating it, and might be a bad practice (that they could then generate huge diffs without having to download them). Probably if it is not cached, the user should get a cached=false or notcached="" parameter, so they know they must &rvdiffto to generate it first.