I think it might be okay in the early stages of a community when they're trying to grow a user base, "build it and they will come" sort of thing. But spamming more content than the community can keep up with is just annoying. I think they should be tweaked to only post a certain number of times per day so there's a chance to get some meaningful engagement on each thread.
I think the best solution would be to have an API affordances for it. Have some sort of historical boolean field that lets you import the content but keeps it from showing up in feeds.
That's way you can move your community's history over to a new home without drowning out other content