Infomaniac:
Whipples and screw blowers are very effecient. Centrifugal blowers - turbos etc are more effecient than roots blowers.
A roots blower does not compress the air inside the blower. It provides the engine more air than it can use so the pressure is increased inside the intake manifold.
A dyno will tell you at what point a roots blower is defeating it's purpose. You plot the boost curve along with the power curve.
At the point the boost is increasing and the power is decreasing shows you where the air is expanding due to heat. So really, the best method is to take the engine to a dyno with a set of pulleys and determine which combination is the best for your application (engine set up)?