Would you kindly elaborate on what you perceive to be the subtleties about IQ? Besides the danger of mistakenly thinking an important factor is the only factor, which is of course a common problem. About predictive qualities in childhood, take a look at [0], maybe this study is more to your liking. But anyway, this discussion is about the predictability of academic success in young children, so please provide a source for your claim that such predictions are infeasible.

[0] Deary, I. J., Whalley, L. J., Lemmon, H., Crawford, J. R., & Starr, J. M. (2000). The stability of individual differences in mental ability from childhood to old age: Follow-up of the 1932 Scottish mental survey. Intelligence, 28, 49 -55