the factory belt is 8 ribs, and with that small of a blower pulley (2.76"), belt slip can be an issue.  with the snub idler that mike has (and i have the same one on my car), the belt almost touches itself, and there is zero belt slip.     
   this set-up made 505rwhp/495rwtq on my car with a pretty aggressive tune, a catback, and a K&N intake, with just a boost-a-pump, and mass air booster--that's it--stock headers/cats/etc.   
   once i added injectors, and made the tune VERY conservative, the car made 486rwhp/480rwtq