A couple things to keep in mind:
Barrel jacks are not intended to deliver their maximum rated current 24x7x365
Barrel jacks are rated with the assumption that they are periodically disconnected/re-connected so any oxidation is scraped off and a fresh contact surface is made.

As for PCIE plugs - max current is HIGHLY dependent on what kind of plating is on the pins. If silver or gold each pin *can* handle up to 5A. Safe max for tin plating is just 3A. Those max ratings also are highly dependent on what gauge wire is used as the wire pulls heat away from the connection. Typically max current only applies to using 16ga copper wire.