Overview:
Why even use C-States?
With todays processors its not recommended to disable C-States completely.
You definitely want some form of downclocking while not in use so you dont have constant (high) voltage being supplied to the CPU.
I have tested the most common forms of C-States.
Test:
I have taken the averages of 2 runs each, the actual file can be viewed HERE
I personally use C1E OFF + C8
-Tqhr