For the 1000W output power supply, it can output 1000 W, while it can draw 1000W/91% 1098W. So it still has some redundency.
I thought that efficiency is related with the difference between the load needs and the power that you need from the wall.
For example, if you have a load that needs 100W, and you have a PSU with an efficiency of 90%, then you get from the wall 110W.
The efficiency is highest at around 50% of the maximum output. It is lower when the output is too high. I run my output around 70-80% of the maximum output.