There is indeed support for this relationship in the literature. There is evidence that attentional allocation and motor performance compete with each other. As speed was emphasized, there was a commensurate decrease in accuracy. The reverse was also found to be true. Her model describes the interactions of three components of performance.
Case suggests that total processing space remains constant across the developmental period.
Case was, of course, talking about normal development. It can be argued, however, that her model has significant implications for individuals with atypical neurological development. One way to think about this is to think back to the time when first learning to drive a car. This activity generally demands the total concentration of novice drivers. Even as ease increases on familiar roads, attentional demands sharply increase when a new driver hits an expressway for the first time.