Each chapter ends with 20-25 review questions. These are similar in difficulty to the mock exams. The idea is to get really good at understanding the material and then attempting the mcok exams.
All three mock exams contain the 1Z0-809 material.
For the 1Z0-810 (upgrade from
Java 7), using the three mock exams as is works fine. This upgrade exam is a subset of the 1Z0-809 so you'll be ok. And the material that isn't on the upgrade exam is things you learned for the OCP 7 in the first place. Which means it isn't new. And is good to review. You don't want to get an upgrade question wrong because you forgot how try with resources works!
For the 1Z0-813 (upgrade from Java 6), you might need more than just the three exams. Because there are some topics that are on this exam and not the 1Z0-809. We cover all of these topics in Appendix C along with 21 review questions there. For more questions, I'd recommend
Enthuware. Also, if you happen to have a Java 7 book, you could use that as well. Keeping in mind some of the topics on the OCP 7 aren't on the OCP 8.
You can get a feel for topic mappings from
the CodeRanch Upgrade exam wiki page. We also have a table in our book mapping the objectives to specific chapters.