Are manufacturer AC cables good enough?

Yes, I believe that thinking is flawed.  Most stock power cords are junk. MFG’s anticipate that you will be using a higher end power cord with their product. They don’t include a good one because it would increase the retail price of their product when compared to competing units that don't include a higher end power cord...which none do to my knowledge. 

Of course subjective listening and opinion is vital too, but equal measure of science is often overlooked.

There is a difference between "subjective listening " and "subjective measurements " the former can be dismissed the later is used along with "scientific measurements" in evaluating whether something has an actual effect.

If Ayre whose top line amplifier sells for around $35,000 tosses in junk power cords then I think I'll look elsewhere for a company that's tested all the parameters that can diminish the performance of their products. For that kind of money I expect a good power cord whose measurements equal or better anything put up against it. 

I challenge all to a blindfolded side by side test. I guarantee not 1 can pick the aftermarket AC cable from the factory cable.  Let the games begin. 

PLACEBO EFFECT:  The more you spend, the better it sounds.  We trust the designers and manufacturers to make really good equipment.  Would they really sell something as cheap to make as a power cord if it made their great design sound worse?  It takes three months to break in.  NO!  It takes as long as three months for your credit card to automatically refuse a refund.  Dealers like profit and reason for you to keep visiting.   An expensive power cord is about as effective as the Scotch Guard on my Tacoma seats that I was charged for.  The unused can of Scotch Guard was in a storage compartment, to be found months later.  If an amp has a crappy power cord, what other cheapness is in the unit?