there are def differences in instrument cables- i have a cable called "Ripcord" made by i believe, MIT, and swapped this out w/ several other brands and there's no denying the improvement and definitionI've never experimented with speaker cables, but I have heard differences in instrument cables, both length and brand. I've had some that make my guitars sound brittle and slightly fizzy, some that make it sound like there is a blanket over the guitars sound, and some that seem to really open up the guitars tone. Same guitar and setup, just swapping cables out between the guitar/amp/Axe.
same w/ speaker cables- the same company made me what they call "GAS" cables and those mothers def thump w/ tight low end (my 4 x 12 Bogner cab alone w/ 13 ply birch buries an 11 ply Marshall cab just for an example of improvement w/ diff materials)
i also had detachable power cables made by MIT for my amp heads, really thick pythons w/ an inline block that had a profound effect on my tone
i also had great results using a power conditioner to clean up the supply electricity, the particular model, by MIT called "Z Series"
all these little touches add up to a better bigger picture
i know what Larry is talking about how you want to then replace ALL cabling w/ the good stuff but the expense is astronomical-so do it where it makes best sense