Do you have a source for any of this? x86 is much more powerful than ARM by watt, being exponentially faster at most math. I've never had anyone seriously propose that ARM is more efficient than x86 at anything then not pulling watts from a Li Ion battery.
Can you elaborate what you mean by "exponentially"?
For ARMv7 vs x86, yes, x86 just destroys ARMv7 (Cortex A15 etc.) in double (float64) performance.
While I do think x86 is still faster vs ARMv8, the gap is likely much less per GHz, because ARMv8 Neon now supports doubles much like SSE. Of course Haswell has wider AVX (256-bit) and ability to issue two 256-bit wide FMAs per cycle (16 float64 ops). Cortex A57 can handle just 1/4th of that, 4 FMA float64 ops per cycle.
That said, low to mid level servers are not really crunching much numbers. They're all about branchy code such as business logic, encoding / decoding, etc. Or waiting for I/O to complete.
So why would you care about math in a low end server CPU if it's not being used anyways?
