It takes about 14A for 18AWG to reach 90°C. At around 11A the temperature will settle down at ~75°C, which is still safe short-term (probably a week of 24h load). ATX standard takes about 1/2 of max, thus 18AWG = 7A carrying capacity. 16AWG nets 90°C at 18A, and ~75°C at around 20A.
The trouble begins with bad contacts (oxidation, loose connector), where the electric resistance rises, causing the power dissipation to spike, producing heat.
Careful what you wish for... You just might get it.