The snarky answer to your question is....get a 15! But it won't hurt to switch out carbs, and you might get a little more speed out of it. And might have some fun at it too. (Look into the reed plates too -- can't speak to that.)
I fooled around with an '83 and '84 9.9 earlier this year, along with a '91. Both the '80s models had what I take to be a 15 carb body to begin with. It's a larger throat (See Leroy's Ramblings on this -- good stuff.) As I recall, there are three different h.s. jets for those carbs -- mine apparently has the middle size, for the Yachtwin model. You would presumably want a 15 carb body with a h.s. jet for the 15. You can buy the jets.
I was more concerned about slow running, as I use them mostly as trolling motors. I found that the carb with the larger throat (and the mid-sized jet) idled down real well -- no l.s. issues. I can't give an apples to apples comparison with h.s. running, unfortunately, but pretty sure you won't lose much on the low end in using the larger carb. Carb and motor have to be functioning right, of course, but that's always the case.
