[Hammer]

The recommended PSI differences are just in the nature of the beasts. One of the reasons is certainly the high torque output at low rpms of the roots style.... along with other issues that others may be able to explain better than I....
Remember that output doesn't necessarily directly respond to boost numbers. Case in point: An intercooled setup can produce more horspower at 8 psi than a non-intercooled unit at 10 psi.

As fas as pullies go... Almost every blower setup I know of either comes with their own set or expects you to use the stock pieces.