Vortech does not rate their pulleys by boost. Quite simply they use many parts in multiple applications. In the case of the Z, the stock Vortech pulley size is 3.33". Most have been getting a peak of about 7.5lbs of boost with this pulley. Though, some have hit higher (mine peaks at 9.5lbs with the 3.33" pulley, but I think mine is the extreme case).
The next size down is a 3.12" pulley and it is, in general, good for about 1.5lbs more of boost.
If you're really adventurous, you can go to the 2.87" pulley like some others have. Requires very precise tuning as you are pushing way more boost than the kit is designed for at that point. It can be done, just be sure to recruit a good tuner.