You're right of course even with a big spread it does, to some extent, give you an idea of likely "performance-in-a-classroom" potential (as compared to actual reasoning ability).
And the GAI gives you an idea of likely "performance-in-a-classroom potential" if the teachers accommodate any issues that may be impacting processing speed and working memory. That's why it's so important that educators understand the importance of all the composite scores. Maybe someday they will (wishful thinking, I know).
