When fetching items, rss.pl puts a hard LIMIT 15 clause on the end of the query. This is to ensure compatibility with RSS 0.91. This restriction is lifted with RSS 0.92, but rss.pl does not recognize this.
The rss.pl script should have a mechanism for identifying the RSS version, and if it's 0.92 or greater, allowing for users to specify their own LIMIT clause on the query.
The templates for this rss script haven't been updated since before we switched to Template Toolkit. Since then we've added the ability to make SQL reports public. I think it's time to get rid of /misc/cronjobs/rss altogether.