your claim that x86 has 8-bit mode is false; the lowest common denominator for x86 is the 16-bit 8086, which you're probably confusing with the 8-bit 8080 which is not x86 compatible
He was probably thinking of he 8088, which was an 8086 with an external 8 bit bus. Internally it was identical to an 8086, and so by any reasonable definition it was a 16 bit chip. It was probably the commonest version of 8086 released because it was used by the original IBM PC.
their attempts to match ARM in performance/W are so far unsuccessful when looking at non-biased benchmark results
True, but to have hope of winning the power/watt race currently, they would have to produce a chip that runs as slow an ARM. If things were static they might have be tempted to do that, but they aren't. Instead Moore's law means a OOO superscalar chip will practical on a phone in a few generations. And with it, the power advantage ARM gains form less complex, slower chips will disappear. Once that happens, the overhead imposed by the amd64 instruction will be so small it becomes irrelevant. Intel seems to be content to just wait for that to happen. Or maybe it's more a consequence of not having a choice, because the complexity of x86 did appear to impact the underpowered Atom badly.
Whatever the reason, Microsoft's abandonment of Windows RT hints that simply waiting will work. Microsoft abandoned RT because ARM simply doesn't have the horsepower, and while an i5 does get over a day worth of battery time. So they have already hit the power budget of a tablet. A phone can't be too many generations off.
But with the Mill architecture claiming a 10 to 1 MIPS/Watt advantage, while having the same raw horsepower as an OOO superscalar core, I can't help but wonder if both ARM and amd64 will loose this race in the end.