If a simple power cords can improve the quality of sound to the extent that it warrants its exorbitant price tag, why would you not (as a designer and seller of amplifiers) sell your amplifier straight out of the box with such a power cord.
The problem is that there is a very good chance that the power cord will be dumped in favor of another. This is why we have always had IEC connections on our products, so it is possible to audition power cords. FWIW, on our MA-2 we actually have 2 separate AC circuits, using 2 power cords. This was done to minimize the effects of the AC wiring both inside and outside the amplifier. That feature was designed into the amp in 1990.
The power cord is never being used as a filter BTW. I can only describe that idea as mythology.
Regarding the readings that Shunyata had in that article, I think it would be very interesting to test some common cords on there, like certain models of Belden. I think that heavy gauge is usually a good place to start- install some decent connectors and you are usually well on your way. However if you think about it, just doing that and selling it at a normal markup will result in a power cord that costs at least $150.00- maybe more like $200...