Against my better judgment, I am going to jump in here. First off let me state that the stock power cables in my system have been replaced, but not with what any of you would consider "audiophile" PCs, just heavy duty equipment grade IEC cables.
There appear to be two ways to look at this, either as the PC being the last few feet of the delivery (looking into the component) or the first few feet (looking back out from the component. Looking in, a well designed power supply should filter out any unwanted ripple or noise and looking out should prevent any unwanted ripple or EMI effects from going out to the hose wiring through the PC. Equipment must pass the relevant EMI standards (FCC class B, etc) It would be possible to measure the differences in conducted EMI using different PCs
Looking into the system, there are also relevant specifications that can be measured, such as PSRR (power supply rejection ratio), transient response and spectral content. I am not saying one way or the other whether any of this is audible or not, but it would still be interesting to see if there is any real, measurable differences in performance between a standard IEC PC and an aftermarket high end cable.
I have access to extensive lab equipment and a shielded EMI chamber with conditioned AC source. Anyone in the DFW area interested in doing some comparison testing (you will have to supply the after market cable)?