Why do I say that? Well, first read this review:
http://jpslabs.com/Reviews2/inwall1.htm
Next, think about this situation logically. How could a fancy power cord make a difference? The electricity coming into your home has traveled hundreds of miles through huge aluminum wires, gone through multiple transformers, and then ends up on a cheap buss bar in your breaker box. How is 50 ft of 10AWG cable from the breaker box to the outlet going to make any kind of difference? The same reasoning goes for expensive line cords. Most people still have normal Romex in their homes. How is 6 ft of ultra expensive power cord going to help the matter? Any company that looks for reviews that proclaim almost magical differences with cables, then charges $2000 for a 6 ft pair is pure BS, especially when a company is using an inferior conductor. Add to that there is yet to be a well-documented and carried out scientific study that proves cable changes are audible, provided they are not such a poor design that can actually alter the FR by a decibel or more in the audible range.
Here's another review for an AC cord from Stereophile:
http://www.stereophile.com//accessoryreviews/817/index.html
As for their credibility, I find them useless on cable reviews, but their speaker measurements are good and consistent.