After an email exchange and a little bit of prodding in the right direction from Yours Truly Mr. Jeff Atwood, I finally understand what's going on and how to fix it.
Right, here’s what is happening. When a feed is added to Outlook, the default settings (which are hidden under the ‘Advanced’ button) are to use the publisher update recommendations.The publisher of the feed can specify a Time To Live (TTL) setting, this is the time that content is expected to remain cached before it is refreshed. Outlook interprets this setting as the minimum update interval. Outlook will not check a feed for new content more often than this minimum interval.
The TTL element is optional and some feeds (such as those from the Stack Trilogy) don’t contain it. When Outlook doesn’t find a TTL, everything defaults to 60 minutes. So in this default situation, Outlook will check for updates at most once per hour.
To avoid this default one-hour situation, it is necessary to instruct Outlook to ignore the publisher’s recommendations (even though the publisher hasn’t made any explicit recommendations, the absence of a recommendation implies a 60-minute update cycle by default). To do this, when adding the feed to Outlook, click the Advanced button and uncheck the box outlined in red in the screen shot above. Note the yellow highlighted text indicates that the publisher has not specified an update policy.
Unticking that box does allow faster updates, but there is another wrinkle. We've removed the minimum interval, but the default settings for the Send & Receive Groups is 30 minutes. So even when the minimum TTL is lowered to 5 minutes, Outlook still will not even try to check more often than every 30 minutes; this is the default setting of the default Send & Receive Group. So, to get full control, it is necessary to uncheck the “Use publisher’s recommendation” box, AND to create a custom Send & Receive group (or modify the settings of the default group). I don’t recommend modifying the default group.
So, create a new Send & Receive group and in its properties, select the RSS feeds that you want to update at the higher rate. To do this, go to the File tab, click Options in the left column (its below Help), select Advanced in the Outlook Options dialog, and scroll down to Send and receive. Click the button to bring up the groups dialog.
The screen shot above shows a sample RSS feed being prepared for updates every minute. NOTE: Jeff recommends updating no more than once every 15 minutes, or you risk your IP address being automatically banned from StackExchange RSS feeds.