The adjustable cam pulley is common use with high-cam, in order to reset the correct idling.
Reduce weight to gain more horse power?
This is not cam-pulley design for.
Gain more horse power by using only adjustable cam-pulley?
Not much, since all the setting is stock, only dyno tune can tell how much horse power has grow.
How much horse power can get?
There is no actual figure for this, it is base on engine condition and the combination of other part. (cam-shaft, exhaust header, intake....)
Make engine bay look nice?
Well.. I admit the adjustable cam-pulley look really nice. ^_^
It is not recommend for inexperience user to setting by their-self.