Do Field Sobriety Tests Measure Impairment?

Every good DWI lawyer is familiar with the last section in Chapter 8 of the NHTSA DWI Detection and Standardized Field Sobriety Testing Manual that states:
If any one of the standardized Field Sobriety Test elements is changed, the validity is comprised. (***)
Indeed, one of the bedrocks of DWI defense is being able to grade not only the defendant’s performance on the FSTs, but being able to grade the officer’s administration of the tests. In some situations, improper instructions or grading can actually make an individual test or even the entire battery of tests inadmissible.
Unfortunately, for all NHTSA’s efforts to the contrary, there are many problems with these roadside tests – even when administered by the book. They do not, in fact, allow police officers to accurately determine whether someone is over a .08 blood or breath alcohol content.
Furthermore, most of the better trained officers will admit under cross examination that there is no correlation between some of the tests and “impairment” – I know most of the specialized Austin DWI task force officers will testify to that, because I’ve asked them under oath.
To the extent that “science” is involved here at all, the only real measurement is between performance on the tests, and being above or below a certain BAC. Which means there’s an inherent argument for defense lawyers in many DWI breath test refusal cases that the client’s performance on the field sobriety tests is not sufficient proof of intoxication.
(*** Every NHTSA Manual I’ve ever seen actually has this section in ALL CAPS AND BOLD – emphasizing that even NHTSA acknowledges how important this is.)