Are manufacturer AC cables good enough?

Gosta: Your rule is simple indeed, but flawed. You base it on a debate from other people, AND, you give it a long time to develop. Why not “solve” it once and for all: try something for yourself. Unless you don’t trust your ears, that’s really the only way to know.

And what “clearly matters”? And why is it different than “a tweak”? More importantly, why do you think this “other people” never “invest” in “what clearly matters”? 

A general rule to follow is the more audiophiles’ hair catches fire the greater the odds are that the tweak works.

Talk to Ayre, ask them does an aftermarket mains cable change the sound signature of their amps, if so why? If one is objectively better why not use it or advise the use of it? What wire do they use inside the amp? Have they tested different mains cables and noticed observable measured differences? If you want to use an aftermarket mains cable I would make sure it has the required safety label from UL or CE to know it’s been tested. For all the blathering about the inky black darks and improved soundstage let's see some evidence beyond testimony. 

Why would a manufacturer ever sell their equipment with a poor power cable or a power cable that makes their product sound poor?
No kool aid here.