Currently in fanless environments (Such as the iPad Pro) the latest CPU, A12X, outperforms Intel's fanless offerings by a good amount.
I would imagine that Apple could build like performing parts if not better using current A12 Tech and don't forget that Apple is already using TSMC's 7nm process. Additionally, Apple could make sure of big.LITTLE in varying sizes to bring large power consumption advantages to Macs as it stands, along with their Neural Core.
Or who knows, maybe they'll just wait for risc-v to to mature to make any sort of switch.
When I say leapfrog I am implying that I believe this list to be correctly ordered and that Apple will not use AMD chips but wait until they can use ARM.
Just idle speculation