Take an arbitrary figure of 100BHP.

20% of that is 20BHP.
Now ask yourself...do you really believe that alfa would not have
'liberated' the extra 20 BHP that these gizmos claim they can provide.

Even the claim itself is unbelieveable.
A claim of 2% would be more realistic (if the system worked - and I have
absolutely no doubt that it won't)

You just do not get 20% increases in power without significant
engineering...