There have been reports that in some embarkation ports, they stick to the arrival appointment time, even for those with priority (including suites, FTTF, Diamond, and platinum). The written policy on the website says that this is the case for all ports, but it seems to be, most often, not enforced for those with priority.
I would get the earliest arrival time I could, just in case. Assume you can show up earlier than the appointment you are able to get and go through the priority line, but understand it is possible they could make you wait for your appointment time.