Anything better than 10 guage Romex?

The Belden 83803 is power limited, Class 2 wire. It is not approved for branch circuit power wiring.

Obviously, persons using any wire should check to see that it meets code, and will function properly, wherever they intend to use it. I ran mine in our apartment in Sao Paulo. PS - Being 12-gauge and used in reasonable runs, Belden 83803 does not restrict current in typical hi-fi applications. Even 14-gauge is fine for runs up to 75 ft. or so.

In the USA Belden 83803 is not approved for branch circuit power wiring per NEC (National Electrical Code).... As far as the manufacture specs, it is designed and UL approved for Power Limited Fire Alarm, (PLFA Class 3)wiring.

NEC 760.121 (A)(1), (A)(2), or (A)(3)

My only reason for pointing out the Belden cable is not approved for branch circuit power wiring is for the OP's and others benefit..... If the cable is used in a manner it is not intended as well as not Listed, approved, and for what ever reason there was a fire or personal injury an insurance claim could be denied by the insurance company.

.

You might ask where in a fire alarm system would #12 wire be used? Horn, or Horn/Strobe circuits...

Can it, legally, be called a "power cord" or "extension" if it has the IEC connection on the end?

Not if it's in the wall and not removable.

The best option is metal conduit, because it's shielded and so less likely to radiate hum. BX is second best, for the same reason. Stranded vs. solid doesn't matter, except insofar as stranded is easier to manipulate in heavy gauges. Gauge does -- the thicker the wire, the less voltage drop. As someone pointed out, wiring heavy gauge wire to a standard receptacle can be tricky. Agree that you shouldn't use the Belden, which would violate code.