The supercharger you're showing pictures of is a centrifugal supercharger, have the worst qualities of roots superchargers and turbochargers. They only generate significant boost at high rpms. This is because the turbo's pressure increases as a square of the RPM, which means if you set your max boost to 8 Psi at a 6k redline, then you'll only make 2 Psi at 3k. Near the redline will a centrifugal supercharger generate the amount of boost you're looking for, but SO WILL A TURBO.
EDIT: In summation: Centrifugal<Turbo<Roots[/QUOTE]
Let me ask a question. I was told that a centrifugal supercharger was basically a turbo driven by a belt. And some do look like a turbo. How close is that assessment?
1995 325I, tinted windows, K&N, debaffled air box, BMW style #104 wheels, Nighthawk low beams, smoked side markers, smoked corners, smoked taillights, M3 style heated mirrors, various interior bits, Bosal cat-back