I am most familiar with the Johnson-Evirude 9.9-15. From 1974 to 1978 the difference between the 2 was mostly the carb.The carb on the 15hp had a internal throat dia of .875 where the 9.9 had an internal throat dia of .625.The rpm's on the 9.9 was 4500 - 5500 where the 15 was 5500 -7000 rpm's.In 1979 they added a shim, part #325038, (one for each cylinder), behind the leaf valves & under the stop plate for the 15 hp, apparently to allow the motor to suck more fuel & air mixture in.
In 1981 there was a change made in the inner exhaust housing for the 15 hp OMCs only. It was a sort of tuned round exhaust housing, allowing the engine to again breathe better, which in addition to the larger throated carburetor, & the leaf valve shim gained about another 10% in overall HP to put these OMC engines on par with the Japanese imports. The 9.9 square inner exhaust housing & no shim for the leaf valve, stayed the same as it was. The big difference in the performance between the 9.9 & 15 hp will be in the last 1/3 of the throttle range where the increased fuel/air flow of the carburetor will be readily apparent with the increase in RPM.
Do not think that you can up the horsepower by simply modifying a existing 9.9 carburetor by replacing the main jet with a 15hp main jet. [-X All you accomplish is that the engine is being fed too much fuel & not enough air to properly mix with it & the engine will then blubber (floods out) at the top end. It can run OK on the lower end & idle OK because it is using the idle jet then, but it will have less top end power than with the original 9.9 main-jet.