Well look at it this way, lets say all the accessories appear to the engine as an inertial load of 300lbs. If you underdrive it by 30%, that's 100lb loss in inertial load. That is a significant improvement even considering the inertial load due to the car is 3500lb, and alot more significant than a pulley that is 5lb lighter :) Most of the gains from installing a UR pulley comes from the underdriving, rather than the loss of weight on the pulley...
Not saying I endorse the UR pulley though, if you search the original UR pulley debate posts I was among the first to object to their lack of dampeners...