Exactly, which meant that 850W PSU's were put into an over-spec situation
Nope. Power supplies are rated on output. At best efficiency would be 90% at high end of the curve. So 900W/0.9 = 810W.
Overspec would be 850/0.9= 944W AC.
And I highly doubt that 850W DC is its maximum anyway.
This review has tested it up to 110% load (934.00W DC / 1032.15W AC) and efficiancy was pretty spectacular: 90.49%