High quality in-wall UL-C2 rated 10-gauge A/C wiring ?

I did testing with a double-run of 14awg romex (that's 2x14awg which makes an 11awg total

Please describe the 'test' in detail from start to finish

Unless you were able to compare the original to the new, in real time, any perceived change is pure fantasy

voltage in the tiny 14awg wire being restricted by the high dielectric constant on the insulation

ROTFLMFAO!!!
@ 60Hz?
Gimme a break!

Yeah, seriously.  I have experienced this same exact thing in power cord conductors (which transfer the same 60hz A/C).

As far as the 2x14awg romex test was concerned:
1. Take two 50 foot lengths of 14awg romex.
2. Take the two black hot leads of both cables and connect them to the same 15A/20A circuit breaker.
3. Take the two white neutral leads and connect to neutral bar in sub-panel.
4. Take the two bare ground leads and connect to ground bar in sub-panel.
5. Run the two 14awg romex cables to outlet.
6. Connect the two black hot leads to hot connection on outlet.
7. Connect the two white neutral leads to neutral connection on outlet.
8. Connect the two bare wire ground to outlet ground.

So, essentially, I am doubling the size of the wiring from circuit breaker to outlet. This is the same thing that happens inside larger power cords.

auxinput

1. Take two 50 foot lengths of 14awg romex.
2. Take the two black hot leads of both cables and connect them to the same 15A/20A circuit breaker.

You'd need to check NEC on this as well as your local code. Also, note that only breakers designed for such a two-wire connection would be permitted.

This is the same thing that happens inside larger power cords.

No it's not.

Even if R were the same, L&C are vastly different.

In the 'test' did you also have the heavier ga single wire connected to the same side of the transformer and routed closely so you could listen to each with minimal delay? If not, there innumerable changes possible on the A/C