Are manufacturer AC cables good enough?

I went to such a test.  About 50% picked the more expemsiver power cord.  We did find a difference in interconnects, but only the very cheapest were picked very often as the worse of a choice.  The others were about 50-50, with mostly guessing.  There was never a consensus, except for the decades old, cheapest crap from Radio Shack.  Basically, any cord with oxygen free copper was deemed somewhat better, but that could simply mean they were put together better, or had better insulation, etc.  One exception:  If your power cord runs parallel to your interconnects, or even crosses them, better insulation/grounding might help. 

How good are the power wires running behind the walls to your power sockets?
I don't think it would make sense to buy expensive power cables if the wires supplying power to your sockets are of regular quality.

I did try and I did hear...and not subtly.

If you did not try in a blind controlled test then you're just offering subjective biased opinion.We ALL have subjective biased opinions when it comes to seeing and hearing the only way to know if something actually works is to remove the biases as much as possible. One of the best examples of conducting an ABX test is done by Floyd Toole when he was at Harman. It's controlled by computer and the results from blind compared to sighted is very revealing. It's and hour long if you care to watch it. 
https://youtu.be/zrpUDuUtxPM

I run a Lindemann, PMC, JL Audio system. Recently upgraded all my power cables to AQ and NRG. Was anticipating a big audio improvement. There was a slight improvement I believe when I switched back and forth between the stock and after market power cables. Not the "Massive improvement" toted by the cable manufacturers and testers of those products. Although there is a huge difference in the quality of the cables look and build. The NRG cable is truly a work of art so I am happy. 

Power cord must be:

1.UL rated
2.Originally supplied, or meeting same specs as original
3.Long enough to reach your units easily ie. no tension
4.Connected to a good quality surge protector (unless your stereo is a cheaper crap than the power cord itself ;) )

Did I miss something?...pls add