if you are having issues with heavy steering, I believe that going up a size on the cog would be the wrong way to go. It may work in flat water
,but IMO it would struggle oin rough weather
due to higher torque. I would go down a size and let the motor
run more easily.
This is a bit over the top but you might consider removing some material from the trailing edge of the rudders .
You might also consider an hydraulic steering
pump. I don't know this boat
at all so take the last 2 suggestions with a grain of salt