If you are spinning 5600 rpms, you are quite over propped. Your motor is supposed to spin 5800-6400 rpms. For a 4 blade, I would try a 17" Revolution4. With your slip of about 7%-8% at the moment, and partially depending on which Merc prop you've got now, it should put you around 6000-6100. That should work quite well for mid range efficiency. A 17" Mirage+ could also be a good choice, even though it's a 3 blade. Which Merc prop are you currently running?