I recently fixed a bug which made it impossible to submit a gzipped feed to Syndic8.
Right now there are over 7,500 compressed feeds in the Syndic8 database. On average, the compressed version of a feed is just 30.42% of the original size. Here’s a graph:
If you are giving out feeds, consider installing and enabling mod_gzip. If your application consumes feeds, be sure to accept gzipped content.