I don't know much about IP Board specifically, whether sub-forum feeds are even possible, but from the screenshot Brian posted that appears to be the top-level, full-forum feed, and deselecting any of the sub-forums would remove them from being published at all.
I disagree about the overload, because it's only publishing new topics, not every post. The number of new topics is much lower than the total "reply" activity. If it was per-post, it would definitely be out of control, but this way should be fine if the limit is raised; topics should only come up once, they won't resurface when they're replied to.