I don't believe they equate those scores as "equivalent", but rather look for outstanding achievement to accompany IQ scores. The SAT does not measure IQ, and is no longer as strongly correlated with it as it used to be, so the question you ask doesn't really have an answer.
That aside, scores in the 700+ range are virtually at the test ceiling, and scores much higher than that are more about perfection than increased ability. The test loses it's ability to find really out there test takers in the late high school years due to ceiling issues.
I'm just basing this statement on their admissions page which states that they accept either a 145 or better on the WISC or a 700 or better on the Sat verbal for consideration for entry into the program. Perhaps 'equivalent' is the wrong word, but they seem to treat the scores as providing similarly relevant information about a candidate's verbal ability.
http://www.davidsonacademy.unr.edu/Articles.aspx?ArticleID=144&NavID=1_33Your point about the ceiling effect is well taken. I did not realize that the SAT had such a low ceiling.
