Marine engine loading during their duty cycle is different, needs to be considered. It is more like a reverse tractor pull That is why most marine engine tuning leans towards the "rich" and less radical than what one might think for auto type setups, need to watch combustion chamber temps, hot spots and the like. I know there is power to be had with ignition timing and tuning, used to make some chump change on the side with that, but remember in a boat there is only one gear so more RPM helps but little MPH there. If you do prop up some then that loads the engine even more, it is up to you.
Watch the plugs as you venture into the experimental zone. Obviously step up the octane as you move the timing up. You also do not want to get the idle up too high as the initial moves up so adjust throttle, etc. Keep the fuel mix rich, things like that, don't let the timing outpace and leanout the fuel delivery mix.
Also keep in mind most drives do not like to liave at 5k and above for extended periods.
