If the primary motivation for attending is the emotional rewards of meeting others with interest in rationality and feeling that you've learned how to be more rational, then yes, a Christian brainwashing retreat would make you glad you attended it in the same way, if and only if you are/became Christian (since non Christians likely wouldn't enjoy a Christian brainwashing retreat.)

That said, as many of us have little/no data on changes in rationality (if any) of attendees, attending is the only real option you have to test whether it might. Confirmation bias would make a positive result weak evidence, but it'd be relatively important given the lack of other evidence. Luckily even if the retreat doesn't have benefits to your objective level of rationality it sounds worthwhile on the undisputed emotional merits.

I think what SilasBarta is trying to ask is do we have any objective measurements yet from the previous minicamp that add weight to the hypothesis that this camp does in fact improve rationality or life achievement over either the short or long term?

If not then I'm still curious, are there any plans to attempt to study rationality of attendees and non-attendees to establish such evidence?