I had installed the pulley set on my Z at the same time as an ATC lightweight flywheel and stage 2 clutch. I can't say as to what difference it may have made in terms of power because I didn't dyno it after, but the combo made the revs insane. It revved like a sport bike. I did go with the under drive version, and did run into a problem with the battery going dead (an Optima red top first, then a Diehard). I don't know if it was due to the pulley or an issue with the UTEC standalone unit, but it did stop when I went to the regular pulley.
You know, I had all this stuff on that Z, and for the money I spent, I would have seen far better gains on a supercharger. I am glad Stillen is developing these performance parts, however, if I go for more power, I am going for MORE POWER. After replacing the bathroom, kitchen, central air unit, and deck so as to avoiding being killed by Mrs Ten*K.
Extremism in defense of Liberty is no vice.