Kent,
Your plan to leave is reasonable, especially if they fit with subsequent parts of your trip. However, a few points to consider based on my experiences with that tour.....
- The group may not get back to Palermo until early evening. As I recall, that last day was packed full of activities, but that may vary from one group to the next depending on Guide preferences. Would that still allow you to make the connection to the Ferry?
- There's typically a farewell group meal (which is usually quite spectacular), but with my group that took place at lunch on the final touring day. I don't know if all groups use that same method, so that MAY be one thing that you'd be missing. Our group had a final get together at a posh local restaurant for drinks and other treats, which was also awesome.
On each of the tours I've been on, there have always been a few people that leave before the "official" end of the tour for a variety of reasons, so what you're proposing is not unusual.
You could also consider taking a morning flight from Palermo to Naples. Depending on the departure time from PMO, you'd probably arrive at about the same time as the Ferry, and could still "hit the ground running". Check Volotea or Alitalia for options.
Happy travels!