"The chain is only as strong as it's weakest link - a fancy power lead won't change your house wiring or the distribution grid. In some cases the meat bag's own sensors are the limiting aspect."
Actually, many "fancy power leads" are shielded...which does two things:
It prevents the magnetic/electrical field that is around the power lead (when actually providing voltage and current) from having an effect on OTHER speaker, power and interconnect cables in/around the back panels of the hi-fi system...esp if higher currents are being drawn, which leads to higher levels of induced signals in other nearby cables...and esp those that are not well shielded (if at all).
And it prevents other cables from inducing "noise" into the "fancy power lead" too...
Of course, there are all manner of ways in which interference can be induced into other nearby products...but there are ways of reducing/minimising this by careful product design.
The trouble is that some brands do seem to have marketing depts who have Ph.D's in snake oil theories...which then makes it look like EVERY "upgrade" solution is "suspect", when in fact some very simple (and not expensive) products can bring about noticeable improvements.