I've just gotten back from training, ran through the course with my XD9 with only a few FTF's (annoying). A friend of mine took the same class with her 9mm XDSC. She ran into a bunch of problems when she was using 40S&W mags in her 9mm, so many so that she ended up just not using them for the rest of the class and stuck with the standard 9mm mags.
Keep in mind - 800 or so rounds over four days, so not a whole of torture-testing. But a lot of rapid, two-shot shooting.