Lightening the pulleys does not add bhp!!!
It adds an equivilent!!
It has the same effect as a lightened flywheel and will, as Moe said, cause the engine to spin quicker, inducing freer reving, which of course gives you access to the power you have later in the rev range, sooner!!
A flywheel is by all accounts heavier to begin with, and is therefore easier to aquire with quite handy weight losses
and it is often cheaper to get a lightened flywheel than a full set of billet pulleys.
Terry, that price is crap. Im paying £189 for a full set for the V6, and thats from Courtney (the rip-off merchants of vx tuneing
)
The thing you have to remeber about light weight pulleys or flywheel is that whilst it'll induce freer reving, it'll also increase idle speed, can reduce fuel economy (driveing style dependant), and will cause a more viscous kick than standard, if you have a habit of charging up towards the redline, before completely lifting off