Should I include a course in which I was strikingly successful in a statement of purpose?
Actually, I suggest that you find somewhere else to mention this, and make the SoP entirely forward looking. The CV and such give your history. The introductory letter can give something of motivation.
But the SoP should be used for just that, what is your purpose going forward. It isn't about what you have accomplished in the past. What do you want to study? Why do you want to do that? What are your long term plans after you earn the next degree? That is what people need to hear in the SoP. Don't assume people will correctly extrapolate your future from your past. Spell it out. Don't waste the limited space on other things best said elsewhere.