Hey Matthew,
I don't have much time to respond right now, so just a few thoughts:
First, I don't know much about this subject except for watching a few videos by Stef and Caplan and rummaging around OpenBorders.info a bit. I started this topic to propose a debate between Stef and Caplan and not to debate it myself, because I quite simply haven't researched it much.
Second, and again I haven't researched it much, so this is just one perspective and I haven't checked all his sources or statements, but here's a bit of an older video by Caplan making some points: https://www.youtube.com/watch?v=GYk00Ufiqb4&t=4m49s
Related to your points, he discusses culture, voting, welfare, and crime (in the Q&A - surprisingly saying immigrants have lower crimes rates. See also http://openborders.info/blog/moral-imperative-open-borders-trumps-pun-intended-immigrant-crime-rates/), amongst other things.
I think he also goes well beyond just purely theoretical grounds, arguing for practical alternatives like immigration surtaxes, tests, etc. Your responses mentions other libertarians who deny more practical concerns, but I don't think Caplan nor I fall into that, so I don't know how that's relevant in this discussion.
I don't see why low IQ is inherently a problem. If an immigrant with a low IQ takes a job no one else wants right now and has a net positive effect, then their IQ seems like an irrelevant variable.
I agree GDP is imperfect, but if one of the main questions is whether or not immigration has a net positive or negative effects, then we'll need some other way to measure that to decide. Are there alternative measurements?
I certainly understand your perspective better now and at no point have you offended me, so your outrage, given your premises, makes total sense. I'll check out those statistics once they're up when I find some time.