There are only two US dental school that I'm aware of that are completely 100% PBL based curriculum and those are USC and Indiana.
I guess to really answer your question it would be to see where those two dental schools rank amongst with other dental schools on the NBDE Part 1 scores. Since we have no access to that information (since it's sacred), we will never know.
There is, however, something to say about PBL. More and more dental schools are integrating PBL into their curriculum. Heck, afterall, even the NBDE Part 1 format is moving in that direction (case and patient based). I've experienced PBL at Midwestern University College of Osteopathic Medicine two years ago in the basic science courses (Biochem and Physio), I must say, it is a unique and effective way to learn the materials.
With that said, please do not look at one set of NBDE Part 1 board scores from a USC dental student and automatically assume that PBL is the way to go. Everyone is different, it may work well with one person, but it may not work so well with the other. Yellowman studied hard for the NBDE Part 1 and that is why he did so well.