The reason for doing this is NOT to blindly increase the khash/s. But to save money by using much less electricity.
(C) results in amazing khash/Watt efficiency: Almost 3 khash/Watt for the whole rig, and 4.0 khash/Watt for the GPUs only.
Very important for people in countries with high electricity prices, and those mediocre weeks with low coin output.
Can I ask how you work out the kH/Watt for the GPU's only?
Sure.
(g)pu (d)isable all cards, then measure the remaining (in my case 81) Watts, and subtract that baseline everywhere.
It's not exact, an idle card still uses a few Watts, perhaps even only single digit - but compared to >200Watts, it's only ~5% wrong.
A discussion of that topic is in
posting #72. Come over if you are interested.