18AWG is perfectly fine for the uses I listed above. Lutro0 use 16AWG because it trains better than 18AWG not because it's actually needed.
Edit: To repeat what has been discussed before:
The Intel Power Supply Design Guide for Desktop Platform Form Factors version 1.3 (April 2013) recommends 18AWG
It's not the wire that is the amperage bottleneck, it's the pins. Since the MiniFit Jr pins are only rated for 7-8 amps, there is in principle no electrical advantage to...