Wikipedia RSS Feed – How to Get RSS Feed for The Signpost

George, you need to check if the publisher's feed is pushing its RSS feed with some push protocol; PubSubHubbub for example.

PubSubHubbub is an open protocol for distributed Publish/subscribe communication on the internet. The protocol extends the Atom and RSS protocols for data feeds. The main purpose is to provide near-instant notifications of change updates, which would improve on the typical situation where a client periodically polls the feed server at some arbitrary interval.

Here an hub hosted on Google App Engine. Give it a try.

You could also register to Superfeedr that handles pushed contents via PubSubHubbub, RSSCloud, XML-RPC or SUP.
Answering some comments, if your feed's publisher does not use this kind of push technology, superfeedr (via polling) guarantee delivery of new entries within 15 minutes of their publication.

As you've noted, there are three categories that divide these up resulting in 8 different feeds:

15 vs 50 - This just tells you how many items appear in the RSS feed. Depending on how busy the group is and how often your RSS reader checks it, collecting only 15 messages could mean missing out on some if 20 had been posted. On the other hand, if the group gets very large postings a 50 item feed could overwhelm a basic RSS reader.

Messages vs Topics - Like you say, the difference between getting all messages or the first one of each thread.

RSS vs Atom - As a user there isn't really any difference between one or the other. Most of the differences only matter to someone programming the feed creation or reading process.