Most things should be tested at much higher loads than the rating. That is the nature of testing. I will say all receptacles in the US are not created equal. We have several grades of the 15 amp 5-15 receptacle and all carry the NRTL stamp of approval but some are marginal at best at 80% of the rated load (the NEC imposed limit for cord and plug equipment) and others would handle far greater than the 15a they are rated for. As in most things, you get what you are willing to pay for. I know I had problems with the receptacles installed in this house when I moved in and I bought a big box of commercial grade receptacles and switches. After an afternoon of screwdriver work my problems went away. Some were just old, others were probably junk when they were installed.
