ok, my 2c is...
i have one installed on my 90 8v. i did so many mods all at once that i cant tell you if it helped. but, i am going to pull it off this spring and see. i did read somewhere that they mostly help at high rps. but my mech. told me he has seen it work on one moter and hurt on another, thinks it has to do with the intake, lifters, carb set up. oh, i got mine from some guy in canada off of ebay.