some guy told me to change for a set of EvoSport pulley kit for 15 HP gain. i got a 99' c280 sport. had anyone here done that? how significant is the increase?

 

Under drive or over drive pulleys don't add hp. Think about it. It would be like saying the transmission makes more hp in first gear than it does in third gear.

They move the hp you have to a different place, so to speak. Every drag racer has a set.