you get a 91% efficiency with a rm1000i that use almost 95-100% of its power

that must be wrong, i have one working at 91%

For the 1000W output power supply, it can output 1000 W, while it can draw 1000W/91% 1098W. So it still has some redundency.

it is actually higher than 1098, with mine the peak was 1200w for power in

If that is the case, then the efficiency of the power supply at 1200W for power in would be much lower than 91%.