I don't think the actual mains cable make any difference to the unit as after all the mains has come many miles before it even gets to your house, so just replacing 1m, or 2m length when after that it once again goes to probably crap cabling inside the unit. Isn't going to make any difference really is it.
But behind your rack is many cables, all carrying different singles. These can pick up nasty interference from them mains cables, and I think this is where the gains are to be had from a better cable or should I say better shielding.
Naim doesn't use any shielding in its cables, as they say it sounds better, well it certainly costs much less.
It also means you have to pay attention to the cables due to all having no shielding and so prone to picking up all sorts of nasty stuff as they go from box to box.
I think as you go up the crazy naim ladder, buying more and more boxes, this becomes a bigger problem, but one you will find inpoves if you do the naim cable dressing or obviously you can just buy cables that have shielding and don't worry about it.
That choice is obviously yours, but obviously naim will say it will ruin the sound, if you don't use their cables.
For me I chose to have better cables when I had all my naim kit, so I could just enjoy the music rather than worry about if a cable had moved and touched another one.