At that point, the fixed load of fans and controller probably start to become a more significant part of the overall power draw.
That is true, with an antminer S9 you have a minimum power draw of around 70 Watts for the controller and PSU no matter the hashrate.
Also don't forget the efficiency of the power supply vs rated output.
Getting the cheapest thing you can find out there vs an 80 plus Titanium may be better then dropping from 750 watt to 650 watt.
There is obviously a large cost differential between the cheap and best power supply, but some of the cheap ones are so bad that no matter how well you tweak your miner someone running one stock but with a top of the line power supply will beat you in efficiency.
-Dave
Just did a bit of a test on a GPU rig.
Using a totally crap no name 1000W power supply running benchmarks was pulling 810W at the wall average with a max pull of 838W
Using a EVGA SuperNOVA 1000 T2, 80+ TITANIUM 1000W running the same benchmark 779W at the wall average with a max pull of 800W
Make of those numbers what you will.
-Dave