OK Gary, your serial number shows a 1978 model which was the long running 10.9 cubic inch block which began in 1962 and went into the 80’s.
Mercury did not offer a 15HP version; they went from the 10.9ci 9.8HP to a 21.9 cubic inch 20HP block.
At a paltry 10.9ci….I doubt that much more performance can be squeazed out of your 9.8. It is already a tuned version of the 7.5HP motors.
