The biggest change with a Cetane enhancer is noise, that's all. You get a softer combustion stroke because of the smoother pressure rise. The engine will seem less rough, slightly
lower smoke for the same power. This can make the car seem more powerful at the higher rpm range, thus the foundation of the perceived higher power.
No Placebo there, its a fact it will smooth the motor out.
Will it increase power, not possible because you aren't increasing the quantity of fuel being burned, and it takes fuel to make more power...
Without the Cetane all the fuel was still being combusted, so it's not like you were losing any performance with the quantity of fuel being injected at that time.
The only way to really get more power and utilize the Cetane increase to re-shape the injection window to utilize those softer combustion rise characteristics and reduction in time to auto-ignite the fuel. This gives a very modest increase in time for burning a slight increase in total fuel quantity.