A challenge to the "measurement" camp

"I saw one brand that was sold with a cord at all, as they assume you will want to use a high end cord with their product "

That seems more fair. You know what you are getting. I mean, you know what you are not getting. They also seem to know their clientele.

More than that: I can calculate what is the cable you need for a system, by having the required length and the DF figure of the Amp.
No measurements are required.
The idea that a cable is a mystery to physics is naive. it is a part of engineering and not a roulette game. Unfortunately, most makers and cable are those easy to make (12-14 AWG) and not the right once (4-00 AWG), harder to make. Your methode, can only tell out of N (number) of measured cabels, which is better. It won’t tell you what cable you really need. Is none of the measured cabels, is among the right calculated values, you trial is no good.

a good 12 ga copper strand like THHN/Home Depot 50 ft spool, will offer equal to any speaker cable , no matter what price range. =  IOW a  12 ga copper, maybe even 14/16 ga  copper stranded,,, all will be equal to even those $50,000 speaker cables. 
They will sound identical, as per opening post,,,
Now as for interconnects,,No , not all are the same, in fact my guess is ~~most~~~ will sound different.
That said, there is no need to spend more than $50 of a  DIY interconnect that is made by the correct cable and correctly made.as I've just discovered. 
You see back in the day,,all we had were basic Radio Shak interconnects,,, ,,there was no discussion,,Now i hear Radio shak/Chinese generic interconnects as trash, next to high tech DIY interconnects. 
Summing up
1) a  nice gague stranded copper = all are identical, no class A,B,C, 
2) all interconnects are not equal/identical. Some are  Class A) superior   Class B)  pretty good and.... class C)  worthless.,,,seems to me the Class A are this new material called copper/coated with silver.
The class C are all those countless 18,20,22 gague chinese interconnects sold under thousands of names past 40 years. = worthless trash. Ebay  has countless sellers listing that trash at $50+ a set. 

10 gague is too thick, disregard, not necessary, 12 THHN is all you need, But my guess if you A/B a  12ga strand vs a  14ga , you could not tell the dif,,I perfer 12 gague as cost is the same as for the 14 gague,,, and 12 ga fits nicely in the speaker post hole. 
Same with power cord DIY, 10ga is too thick, 14ga might be OK, but ideally 12gague seems absolutely perfect in my DIY powercord. 

What thickness is right?

Well, the only cable property that matters is it's resistance.
A cable resistance equals to the material conductivity constant (cooper),
multiplied by its length (in meters) and divided by its cross section (mm sq.).
This resistance shall fit so it will be way smaller than the Amp's DF. (Output resistance). 
Usually, the Amp and length required are a given. 
So what left to play with is the thickness.
Today SS good Amps have a DF of 200 and above...
PASS LAB = 200
EMOTIVA = 400
Luxman = 700
Tube Amp have a low DF (20 and less) so it's insignificant.
D-class tend to have a DF in the thousands: as high as 4000 and more.
Tell me what is your Amp's DF and what length you need, and I'll calculate for you the optimal thickness (in # awg).

A test been conducted two years ago, by myself, and those who pioneered it, ended up very happy. The old cable (no matter how expensive it was, and some were as high as $17,000 a pair, went into storage. The DIY offered ended up (mostly) under $100.-