I worked at IB schools overseas for a long time, DD was not in school at the time, however, looking at it from a teacher's perspective, and having friends with GT kids at the school, I would say that if it is done correctly, the IBPYP program is great for gt kids. The marking is all based on rubrics, so kids can really go in-depth on an assignment, and there are a lot of cross curricular units that work well with gt kids since they tend to see the connections and are curious about them. The program teaches critical thinking skills and problem solving skills much better than the regular American curriculum.
I wish we lived near a PYP school so that my kids could go there, in fact, since we are thinking of moving at the end of this school year, we may look for that opportunity in our new area.