I go on recommendations from Real Hard Tech.
I don't do any power supply testing
and if you would read bellow the list there, they decided to switch the nvidia chips to the values "recommended" by nvidia instead of real word measurements like they do for amd.
my guess is people got burned using their measurements because of bad quality psu-s (bad quality 300W psu + gtx 770 = bad) so they had no choice but to agree with nvidia and their recommendation page. there's a reason a 600W psu is recommended and that's to take into account cheap manufacturers that brand bad quality/low power psu-s higher. (i saw psu-s that were "600W" but only 300W on the 12V rail for example)
not the case with the xfx we're discussing here. it is a good quality psu that does deliver its rated power. only way to get close to the limit it has is if the system is overclocked where the extra 20-30% wattage brings it closer to its limit.