This is a fully minified package of the Superfish distribution. (I used YUI Compressor, see readme-min.txt.)
For the two Superfish files Thematic uses, the gain is 3.5KB: from 7KB to 3.5KB.
The difference is much smaller when the files are gzipped, but there is still a difference even then.
The way I see it, minification in such cases is not done to reduce bandwidth usage, but mostly for user experience. Bit by bit, such small gains accumulate, and often the overall difference is large enough to be felt by first-time visitors, who don’t have the files cached.