alta, I am happy you like the tool!
Regarding 'generic': It's often up to the webmaster whether to offer RSS feeds for his forum. For instance, Mobileread has RSS feeds for
every forum section. Returning RSS feeds based on customized keyword searches sounds useful, but causes some serious scaling issues. As long as you restrict the number of feeds, you can easily cache them for use by multiple people. But if you offer keyword searches, it's possible that your server has to great hundreds if not thousands of individual RSS feeds, definitely putting a lot of stress on any server (bandwidth and CPU).
I am still not sure about refreshing the feeds more than every one hour. So far word hasn't spread much of this tool - but if more people start using it, one hour might be the best frame to limit stress on the servers. Regarding duplicate posts - that issue is most likely related to your feed client (GN) which doesn't keep track of what items you've deleted before and once you refetch the feed, the previously deleted items come back.
update: I've changed the update frequency to every 30 minutes. Let's see how it goes...
Best
Alex