I used one on a mustang actually(the model i had was for a supra tt, WAYYYY before I ever owned one). With it off, it would barely turn the tires over. With it on, definatly a SOTP gain.
That being said, there is noway I would install a product on a turbo car that messes with the timing. "might" be okay with good gas and stock boost, but still too iffy for me