The biggest impact I heard was with my CD player. No difference with pre-amp and very little if any on power amp.
Point I forgot to mention - assuming there's a fixed budget and you could only add 1 "high-end" PC at a time....where would you add the PC and in what component order. On the digital side, I have put my Transport and DAC to the test and the DAC had a greater performance (sound) increase with the high end cable then that of the Transport. Components used were the following: Denon 2800 MKII, Bel Canto DAC, Wolff Carbon Sourse power cord.
It all comes down to the weakest link in your system and some power cords may work on one component but not as well on another. For example, I use an Elrod EPS Sig 3 that I like on my amp but not as much on my CDP, DAC or preamp. I also recently compared a power cord that was a cheaper one versus a higher price cable in a buddies system and mine. My cord in his system was out perfromed by his power cord but in my system, my power cord just sounded more real, go figure.
So as Marakanetz states, find or try out a well know power cord that sells used on Agon that you can resell for about the same money and try it out. There are plenty of people selling power cords these days that also offer a 30 day return policy.