All the processors you mention are the exact same part
set to run at slightly different speeds .
A task like video encoding will scale perfectly with cpu speed [ assuming theyre the same cpu]
If it runs at twice the speed it will get through an encoding job twice as fast . Obviously the margin between 2400 and 2500 is smaller than that .
The 2500K is the same as the 2500 but intel leave it "unlocked" so you can set the speed even higher manually . You void your warranty of course but they are capable of a decent speed boost without harming them
To use overclocking you have to have the right motherboard as well