Building Out / Efficiency
I've basically read through most of this Hardware forum, but I was wondering if we could talk about efficiency and scaling on larger systems.

Obviously there are a lot of parameters that could be maximized: Space, Performance, Hardware Cost, Temperature, Power Use, Reliability

Assuming physical space wasn't a premium, and Overall cost (both hardware and power use) per performance was the main consideration, what system designs are the most effective.

General things I have learned from reading past posts:

-with node computing, system design has shifted out rather than up

-power supplies are most effective at around 50% load

-power supplies get exponentially more expensive at a certain point

-3-4 GPU rigs seem to be relatively "easy" to put together with many motherboard / case options--not yet sure whether this makes it more cost effective

I know in the recent past it was a even harder than now to shove more than 3-4 GPUs in a system due to bigger power draws and thus higher temperatures.  This seems to have mitigated somewhat with GTX 1080 only consuming 180W.

Anyway, I am not planning any system in particular--I was just wondering how to hypothetical scale efficiently.  Any metrics advice or comments would be appreciated.