Daniel, if you do write something like this, make sure to add a setting for the update frequency. Since this is ajax, you can even add a way to shape the frequency based on the server load, dynamically. The higher the load the slower the update time.
Without this feature this would make the servers to crawl even on the small-medium forums - if the users start to use it.
Hints: What happens when the number of returned topics is greater then 15 sec. The script is still loading up the topics in the table and a new request will be made. This would cause some parallel insertions, a total mess. Therefore, you need to write your script to don't send an update request only when the "cache" is empty.
Two, I haven't looked at the other script to see how it handles the topic queue, but topics that go over the bottom of the table should be removed from the page. Think DOM.
That's it for now.