The LM is fundamentally a different kind of measure than the contemporary deviation tests, generating a different metric at the end of the process. We have some data on the relationship between the LM and more recent tests, but it is necessarily limited with regard to HG+ learners.
1. The LM is what is called a power test, for which a somewhat simplified description might involve quality over quantity, and depth rather than efficiency. So it deemphasizes speed and memory.
Elsewhere, the prominence of cognitive efficiency (working memory and processing speed) in standard contemporary measures has been discussed at length.
2. The LM generates a mental age (ratio IQ), derived by comparing the examinee's absolute performance to the median performance by age obtained by the standardization population. This is how the sky-high scores sometimes reported can be obtained. If you score at the level of a 16-year-old (the maximum reference age used by the LM, on the assumption that cognitive development past 16 is negligible) when you are 7 years old, your mental age-based IQ comes out as 229. Of course, if you are tested when you are 10 or older, your IQ will be capped at 160 or less, which is no higher than the contemporary tests, but without the Flynn effect (norm obsolescence/score inflation).
Current tests nearly all use the deviation IQ, which is based on transformations of the z score, comparing the individual to their age-matched norm group in terms of how many standard deviations off the mean their performance falls. Or put another way, their ordinal position compared to age-peers.
IOW, the LM aligns learners to those who perform at the same level, and then derives an IQ by comparing their ages, while contemporary tests align learners to those who are the same age, and derive IQs by comparing their performance.
Allowing some to qualify by using deviation IQs, and others to qualify with ratio IQs, would be comparing apples and oranges. The limited available concordance data for the HG+ population supports the contention that they are related--but not the same kind of metric.
And yes, Pearson is fighting a losing battle against those who illegally resell Wechsler kits.
