Would adding an underdrive pulley be a waste of money having a turbo or would that assist in spool up due to a smoother spin up of the engine?

 

Underdrive pulleys would work the same as NA with the turbo because the turbo does not depend on the belt speed for its power. I am not sure if the turbo will spool any faster or not but you should get a little better throttle response and low end performance. Less drag on the engine will mean more torque available to the rear wheels.