What kind of actual performance gains can be had from propper jetting? The plug colour I have know is a dark chocklate brown, would I gain as much as 200 - 300 more rpm by jetting a bit leaner or is the risk of dammage not worth the extra speed? (Dumb question)

It doesn't sound to me like you're too far off now. Are you checking the plug color right after a hard run without any idle time? Even a short time idling in to the dock will darken the plugs.

If jetting is too rich & you leaned to correct A/F ratio you could gain 200+ rpms. If jetting is too lean, you may gain 200+ for a short time before the motor melts down. Key point is correct jetting from being too rich. I remember where Hot Boat had a motor on a dyno & motor gained 85 hp in getting jetting right on.

Plug reading was done after running at 4500 rpm for 10 minutes or so but not held wide open. I shut the engine down right away and checked plugs