I'm not a guru, but I can at least tell you the little bit that I know while you wait for the gurus...
The LM is a test that does have a longer tail and is generally the test people are talking about when they say someone has a 200+ IQ. The tests that are currently widely accepted don't generally go higher than 160, though extended scoring can potentially stretch that a bit.
The LM is, however, an outdated test that is not widely administered anymore and is not generally accepted for entrance to programs like DYS. (Of course, I realize your DS is already in DYS, so that's not really an issue.) The big problem with outdated tests is that scores tend to be higher on them than they should be because of the Flynn Effect. Proponents of the LM for use with HG+ kids dismiss the Flynn Effect as a minor concern, claiming that it boosts scores by 10 points or less.
On a personal note, I considered giving the LM to my DS9 back at the start of our journey, mainly because the LM is the test I was given as a kid, and I thought having the same test with the same scale might help me to understand him better. How is he like me? How is he different?
Ultimately, I decided it wasn't worth the money. I think living with him has taught me more about his quirks and his needs than another test, particularly one that really isn't going to be accepted by anyone for anything. I suspect achievement tests like the Explore and the SAT will prove more useful to us, and they are a lot cheaper!
But that's just my take. YMMV! Fer sure!
