In my opinion you need a very expensive system to warrant new mains cables. The theory is that the cable introduces less noise into the power supply and so less noise is fed into the DC path. While a low resistance cable may produce less noise the amount is tiny, probably a few pico volts less. Ok it may be screened better, but the rest of your house is not screened and any interferrence will just past through your mains cable (unless it is also filtered). In the big scheme of things there is probably little to be gained by buying expensive mains cables. However it's just my opinion and in the end of the day the good old advice of go and listen to one and decide if it's worth the money probably is the way forward.