Exactly, which meant that 850W PSU's were put into an over-spec situation
Nope. Power supplies are rated on output. At
most efficiency would be 90% on the high end of the load curve. So 900W AC *0.90 = 810W.
Overspec (>850W DC) would be > 944W AC observed (850/0.90 = 944W AC).
If the power supply was less efficient that would mean the DC load was actually LOWER. (i.e. 900W AC observed on a 85% efficient PSU would be only 900W AC * 0.85 = 765W.