Sounds to me like you solved your problem. Oil soaked plugs will do it every time. I was going to say that with that motor on that hull you should be turning with a light load in the neighbor hood of 5500-5700 at optimum for that motor and should have been turning the 16 at about 6800. If she still only pulls 5400 max trimmed out I would seriously consider running a 19 to got max rpm up around 57-58 then loaded she should drop to around 5500-5600. Good luck. Believe it or not sounds like about ready to just go fishing instead of working on it.