if it was a 9.9 johnson you could get a 15 hp carb put on it, if the motor was a 1974-1978, but other wise no, a 15 hp carb wont fit on the 8 hp. your 8 hp is a step up, of the 6 hp johnson, same engine block "I think".
If it were mine I'd buy a 15 hp, then it will go close to 30 mph. and the...