More difficult is somewhat of a relative thing. I would say that these exams tend to focus more on some of the difficult combination, permutation, and probability questions than do the actual exam. Are they more difficult exams overall? Tough to say. If these disciplines are tough for you then yes. However, the GMAT itself tends to have some very difficult problems itself.

Overall, they are quality representations of the exam. The best representations however are the 2 tests from the GMAC. Also, don't stress too much on individual exam results. Your focus should be to continue to fill in any gaps in your knowledge base as you come across more weaknesses.

Good luck, sounds like you are on the right track.