Doubt Apple is moving to anything other than ARM, given how much they have been involved with the development of ARM throughout its history.
There is talk of the A12X having large performance increases putting in line with desktop chips.
However, performance on the whole is an extremely complicated thing to measure, and individual benchmarks have the habit of being cherry-picked. So I retain a degree of skepticism there.
Probably won't know just how ready ARM is for desktop until I see them in daily driver workflows like mine, which they obviously aren't yet.