One does not need to do any changes to the gearbox at all to change RPM. By changing the pitch of your prop, this will alter your RPM at WOT. (Wide Open Throttle). As for power, nothing. An outboard delivers max. HP at manufacturers recommended maximum RPM. So, you need to know the max RPM of your outboard, and are you achieving this at WOT. If so, you are about there.