Interesting question. I don't have your answer(s).
But, like you, looking at part numbers, the camshafts don't match....the spark plugs are different....
the main emulsion nozzle is different and the 9.9 has a #48 low speed jet while the 15 has a #52. Both of which aren't available for either outboard
Also, you wouldn't increase valve clearances for "more air". If anything, that would be reduced. But I wouldn't recommend doing that unless you have the specifications. I don't have that information.
The heads, valves, rockers and springs are the same for both engines so....they're close for sure.
Not to rain on your parade but I don't think trying this will work. The cams are probably ground slightly different for one to handle the increased fuel that the larger jets would provide. And, I'm guessing that the ignition timing "map" for spark advance in the CDI will be slightly different as well.
They haven't offered replacement CDI for so long they aren't even listed in the parts pages to compare numbers.
You could have everything lined up mechanically but, if you don't fire the plugs on time it will defeat the process of doing what you're trying to do.
ie: making more power when it's needed most.
If you could get another cam, the carb jets and a CDI you might be in business but the only thing available new is the spark plugs.
Good luck though.