Asked the same question, Mike Bryant replies: "Unlike in small systems where it is key, in big systems, which smartphones most definitely are nowadays, the instruction set used is far less relevant with only 10% to 15% of power dealing with it. So even if ARM waved magic dust on their instruction set, which isn't perfect anyway, they can only optimise 10% to 15% of the power used. More power goes in moving data around the chip, and most goes to moving data on and off the chip to memory and peripherals."
"ARM programs can actually use a few percent more codespace than Intel but let's say they are the same," adds Bryant, "next of course the data either processor has to be moved around - be it pictures, voice data, HTML or just the call set-up protocol, is identical for each processor. Finally the video data has to be transferred onto the display and for all the hype ARM's Mali is about the same as GPUs from Intel or Nvidia, though there are slightly more efficient solutions from Imagination Technologies or Qualcomm's Adreno."
"So for a pure ARM system against a pure Intel system, 85% of the power usage is independent of the processor instruction set and dependent purely on the process technology," concludes Bryant.
And that's why TSMC/Apple are pushing to 20nm and it won't be cheap
""So for a pure ARM system against a pure Intel system, 85% of the power usage is independent of the processor instruction set and dependent purely on the process technology,"
most of the power is used by lighting the screen
NVIdia has DIDIM which reduces backlight power by 40%
which is independent of process technology...
and if it was just about process technology
INtel would have NO need to acquire 3d patents or a 3d-chip company...