I think you are confusing an ISA and an implementation. There's absolutely nothing about the instructions that comprise ARMv7 or ARMv8 that make them more power efficient than current X86 (well X64 + AVX and well, you get what I mean). This efficiency advantage comes at the architectural level. For example, Krait, A15 and Cyclone all target much lower TDP levels and higher efficiency than Atom or Bobcat/Jaguar as with added transistors for extra computing power comes the cost of efficiency. There's absolutely no reason why someone like Qualcomm couldn't produce a Steamroller/Broadwell class ARMv8 implementations, however they would also run at Steamroller/Broadwell level TDP levels. The inverse is also true (AMD/Intel launching Krait-class X86 hardware), however neither is likely in the near future due to experience. Either of these scenarios would represent uncharted waters for everyone involved and despite all the talent at ARM/Intel/AMD it is just REALLY difficult to start from scratch on something like this. This explains why you see the ARM sector slowly creeping up in power (both computing and draw) while Intel and AMD creep down. The battleground will actually be fought somewhere in between the "comfort zones" of both sides, which my crystal ball predicts will be in some tablet/convertible form factor and/or in servers.