While I disagree with koyoot args about 7nm, I concede him about Apple need to switch from Intel on high-end Macs in a hurry and mainstream Macs ASAP AMD has a viable solution (Zen/TR/Epyc advantages only on desktops, on mobile seems at least another year for Apple tied to Intel).The Xeon-W in the iMac Pro is a big upgrade from the five year old CPU in the MP6,1. The higher current Xeons are even bigger upgrades. (12 core to 28 core)
Apple, as now can replace Intel on iMacs 27 and Mac Pros & iMac Pro switching to Ryzen 2, TR2 and Epyc respectively, Ryzen mobile APU are not in the same segment as the Intel CPU on the MB/MBA/MBP/iMac21, only except could be the Mac Mini (RiP), at least no AMD product in public roadmap, its rumoured a full ZEN APU is on the works with 8cores and 16-32 vega cores good to eat Intel's i7-8809G.
There still too many factors, Intel also is know to be developing its first Discrete Gaming GPU, but seems it is an 2020 product at least.