Not going to happen. There's a performance hit that almost certainly comes along with emulation. Battery life is likely to get worse with x86 Windows emulation on ARM.
Also, by the time Intel stopped pushing x86 in mobile devices they were producing chips that were competitive with ARM chips when it came to power draw.
People forgot this. It was price that killed Intel in this space. They couldn't get the price down to ARM chips. They subsidized hard to get even close, but it ended up just costing them too much just to have even a presence.
In any case, many users may just be wanting to run older enterprise apps, and so even then, with the translation penalty, they'll probably be quite snappy.
My argument is that it's not going to be as power efficient as using x86 natively. There's no getting around that.
From what I read, doing binary translation from x86 to ARM is actually not that hard since ARM has many more registers, so I'm sort of hopeful they get decent perf from it.
I'd suggest it has more to do with the size of the silicon wafer rather than the performance.