i'e heard that undrive pulleys are not advised because they dont power your accesories the same. therefore your alternator isn't working as it's supposed to due to less rotations

a light weight crank would help lower the loss of hp because it reduces the amount of weight that the pistons have to turn. If you wanna do the lightwieght thing and see gains you need.....

Light weight pulley
Light weight rods,spring, and rockers
lightweight pistons
etc...

if you put all that together wou would see very noticabe gains but it would be exspensive and who has the time