Do good power cords have to be stiff?

Unless your equipment is very poorly designed, I don't think you should be able to tell any difference between any power cords, assuming a decent construction baseline.  I can't tell a difference in my system, and the only blind test that I know of confirmed my experience by a guy who has a million dollar system. . . But buy whatever you think will work for you. . .

@carlsbad

12G or 14G should suffice for most: 15A outlets have 14G single strand copper, whereas 20A outlets have 12G single strand copper providing power to the outlets. That’s your bottleneck: having 10G power cable feeding from a 14G wire shouldn’t do anymore that a 14G power cable.

@dekay 

I added my components to my virtual system, if that's what you mean. . .