Here's my recent experience with RCA cables.
I wanted one for my bedroom to connect my Sonos to Audioengine 2 speakers. There was a lot of static noise when I first tried a cheap £2.69 cable (I had not even begun to check audio quality). I contacted the ebay seller, who sent another one out. Same result. I then ordered a £16 cable which was better built. No static noise this time.
I concur, I replaced a pair of rca cables with more expensive ones and noticed that the interference caused by my mobile phone was gone. I do think that with a revealing setup cables might make a difference, but I do not own one. I can confidently say that a better built cable reduces interference.
In a nutshell, I can't comment if there's an improvement in sound, but better built RCA cables definitely eliminate static. Of course, my view about HDMI cables are well known; I haven't noticed a difference.