My intuition tells me the difference in caffeine content will be slight to non-existent between regular shots and ristretto shots. But perhaps I'm not adequately accounting for the reduced volume. And I don't actually know the little details that might contribute to an educated guess.
Which part of the pour has the highest concentration of caffeine? What are the temperature and pressure ranges within which caffeine extraction is most effective? Does the color of the crema indicate anything about the caffeine levels in a single shot?