Well, I'm going to stick myself out here and tell you that I have spent money on after market AC cords. Because, especially with my CD player, they made a subtle but important improvement in sound. I did not spend $8000, but $80 for the amp (from Signal Cable) and $35 for the CDp (a $110 Audioquest NRG-2 off eBay) With the CD player, a Rotel RDC 1070, it deepened the soundstage (made it more 3D) and improved the bass definition and extension. It also allowed the player to extract just a bit more detail. For instance, on track 7 of Freddy Cole's "Music Maestro Please", it made the difference of being able to hear the saliva "tick" in his mouth as he began a new phrase. With the stock cord, that very detail disappears- can't be heard.
Now, I realize by posting this, I'm opening up myself for attack (I'm an idiot, a fool, a victim of some industry wide corporate conspiracy). But the truth is, I think for myself; I don't read Hifi magazines and I often think the guys at my high end shop are full of it. I listen to what they say, and what you guys post, then I form my own opinions. And my opinion, which is supported by my experience is that in my system, an after market AC cord can indeed be heard.
(BTW, I do have a dedicated outlet powering my system-I hooked it up myself during a vacation for about $30 in materials)