I think the question is whether you can pass both OCA and OCP exams after just studying the OCP books. I would say that is something only you yourself can answer. Having reviewed the Certificate Objectives for the OCA do you think that there is nothing in the OCA prep literature that you don't already know?
In general I'd say if you're going for an exam, then it is wise to study the material covered in that exam. If you already know a lot / all of the material then your study time before you feel comfortable enough to take the exam will be shorter.
Nobody but yourself can advise on whether you are suitably prepared for any exam.
Exam Title: Upgrade to Java SE 8 OCP ( Java SE 6 and all prior versions)
But to answer the original question, you can read up on the topics covered in this upgrade exam and then take a couple mock exams that are specific to this exam. That should help you avoid spending time on the boring stuff that you already know about