no matter what kind of engine it is, it's pretty unlikely that you'll get a whole 5 horsepower just by putting on a lighter crank pulley. That's the same type of myth as the DC Sports tube headers giving 10 horsepower.

You'll notice a gain, but prolly not 5hp. Another thing you wanna check up on is that the pulley is very well balanced, cuz I'm pretty sure that you'll lose your harmonic damper when you swap pulleys.