There is plenty of OC room too. It might not even be possible to OC enough with only air cooling to exceed that PSU's rating.
Ok... The HX750 can give max of 600w at 80% of time, so 600w whenever we need it.
How hard can it be, this never ends!
PSU efficiency means what percentage of input energy it will be able to convert to usable low voltage range for you computer.
Thus a PSU with 90% efficiency will consume 1.1 times of its rated power from the wall when producing the rated power.