I graduated from an IB “Middle Years Programme” in 1997 and a “Diploma Programme” in 1999. Both were as you’re describing, a subset of students at a larger school participating in the IB programme. I think it was absolutely worth the extra effort. For the most part, the teachers were interested, engaged, and seemed invested in the program, wanting it, and their students, to succeed. The IB teachers only taught IB students - I’m not sure if that would be the case for the school you’re considering, but if it is, I think they make sure to pick very good teachers for the programme, so you’re getting the best ones in the school.
The other students were almost all nerds like me who loved and wanted to learn, and while we were competitive with each other, there was also an atmosphere of teamwork and “we’re in this together” that really helped us all. What I liked best was learning how to learn, how to ask questions, how to judge information and eliminate irrelevant fluff to get to the core of what you need. When you understand the how and the why of things, instead of just cramming for the next test, a whole lot more stays with you. And that’s what the focus seemed to be.
Did it help me academically? I think it helped me transition easily to university, because I’d already been writing long essays and thinking critically and doing advanced chemistry and math. I was used to a heavier workload, and so it wasn’t a shock. It probably hurt me in terms of GPA (well, Quebec’s version of a GPA), because we weren’t weighted differently because we were at a “tougher” school. Where I went to school, instead of a GPA, they use a calculation and give you a rating based on where your grades fall relative to your peers. There was supposed to be an adjustment made to account for the difficulty of the program, but I didn’t see that happen. So, because our class averages were so high, something like an 88% average was, well, average. But I don’t think the grading system is the same in the States, so maybe that won’t be a problem.
Bottom line: it’s hard, it’s a lot of work, but it’s a different kind of learning that will stay with your daughter and help her to think. IMHO, totally worth it.