Sounds like you are talking about a lightend pulley set, not an underdrive pulley set. yes you can see hp gains, I had underdrive pulleys on my mustang.
Lightened pulleyw work like this. The less effort the crank has to make to turn the engine over, the more hp you will make. Thus by reducing drag on the serpentine belt and making it easier to spin the belt and crank pulley you can gain hp. You mostly see the hp gains in N/A cars because the motor is doing all the work in making hp. You won't see many gains in turbo cars except for a quicker spool. Not saying you shouldn't get the. I am putting on a lightened crank pulley on my car.
Lightened pulleyw work like this. The less effort the crank has to make to turn the engine over, the more hp you will make. Thus by reducing drag on the serpentine belt and making it easier to spin the belt and crank pulley you can gain hp. You mostly see the hp gains in N/A cars because the motor is doing all the work in making hp. You won't see many gains in turbo cars except for a quicker spool. Not saying you shouldn't get the. I am putting on a lightened crank pulley on my car.