It's been a long long time since I have visited. Looking over the posts, this one sort of caught my eye. Diminishing returns are at the beginning of OCD. To be rational about the concept of upgrading any/all cables, it doesn't hurt to try and put in perspective what you hope to accomplish. Basically, I try look at it this way>>> What is the job of the particular cable being considered? In the case of AC power, I might skip by a power cord used to power a TT. Spinning a platter. It doesn't get more boring than that. This is always in consideration of there not being sort of problem to begin with.
For my liking, a power cord must be capable of carrying peak current needed for the component. This is especially considered for say a power strip that may feed multiple higher current equipment. Often overlooked and important to me is the actual connector pressure being placed on the prongs of the cord itself. Remember those crappy AC wall outlets that couldn't even hold onto a vacuum cleaner cord? Connection is big. Personally I get picky about the prongs having holes in them. Would rather have a solid prong. Not meant to be last on the list, but shielding can in many cases be as important any of the above.
Interconnects have the same requirements stated above where applicable, but also there are cases in which additional factors should be considered. Phono cables come to mind, as capacitance is to be low, and shielding is to be good enough to keep out the nasties. I am in the camp that shorter is probably better and neater. Lastly, for interconnect cables, realize that you are attempting to protect a small or very small signal to begin with and so everything matters! A loose connection, or even routing of the cable will show up in hum or noise.
Of course, I could go on a lot longer, but consider this my re-introduction to the site.