Does Dr. Gross' book not provide somewhat of empirical evidence to the benefits of acceleration based on the LM? After all, this seems to be the only test she cites for these children whom she follows longitudinally.
I am sure that AEH will have a better response, but what I would say is that Dr. Gross clearly outlines is that for measurably exceptional or profoundly gifted children there is clear empirical evidence for the benefits of acceleration, academically AND socially, evidence of the difficulty in dealing with educators AND (importantly) evidence for the risks of NOT accelerating (or otherwise providing adequate education).
Her argument is that measurably exceptional or profoundly gifted children are not "spurters" and do not "level out" - unless you willfully suppress their development. Their chances of growing up to be socially well adjusted are inherently tied to their successful intellectual development (relative to their personal potential, specifically not their adequate intellectual development relative to their age peers).
The SBLM is the tool that she had available to her at the time with which to measure... The SBLM is no longer an acceptable tool with which to measure (30 years later). Newer tests have not been designed with gifted children in mind, they are frustrating for experts and parents of gifted children (especially at the tails). But they are the only tools we have that will hold any sway with schools in terms of IQ / "potential" or "ability", the gift vs. the talent, as Gagne would say. Despite the frustrations with newer tests, there ARE children who are scoring at the extreme tails of the WISC-V, extended norms have been made available. That said, it seems that most organisations serving the "profoundly gifted" will simply accept an IQ score of 145+ on a modern measure, as there is no SBLM replacement today.
One of the issues in how differently the SBLM and modern tests work for gifted children seems to be, from my inexpert reading, that the SBLM did not really weight processing speed or working memory the way modern tests do, AND did not penalise a child with a specific strength. From what I understand a globally gifted child could do very well, but a less globally gifted child with a massive verbal or math spike could also do very well. And there was only one final number to describe that outcome, regardless of what their personal strength was, or how "even" they were. On more modern measures children who can score 160+ on FSIQ will be extraordinarily rare, as so few are uniformly gifted in all areas of modern test (including WM and PS). But the WISC-V does provide multiple ways of describing a child with a significant strength in a particular domain and many (most?) relevant program are using these options as acceptable measures of giftedness (VECI, NVI, EFI, GAI, EGAI, QRI, etc)
Tests of achievement are many and varied. Gross used multiple forms of achievement testing in her book too. She repeatedly references school's utter unwillingness to acknowledge children's strengths even in light of both IQ AND achievement measures such as the SAT. Though, from what I understand, not being American, the SAT was also a proxy for IQ 30 years ago, much more so than it is today.
