Hello,
I’m planning to upgrade my 21.5 inch sometime this month, Thought I would mention that the wattage of the PSU is only 185 watts. If you have any variant with the AMD Radeon Pro 560X (75W) and install the 8700k (95W), you may be inadvertently putting unnecessary load on the iMacs power supply. In saying that varies articles do state that at max load the CPU should only draw around 87 watts.
That combined with the rating for the X570 (75W) means that at peak, want to emphasize the PEAK part. Your max load wattage will be around the ballpark of 175W. Its less 170 watts, but added a few watts for ram and HDD and also increases with fan rpm.
If you have the Radeon Pro Vega 20 with 4GB of HBM2 memory, you may be over that threshold as it uses 100W opposed the 75W of the 560X
I will let you guys know how I go with the upgrade once i have completed it.
Sources:
https://www.anandtech.com/show/11859/the-anandtech-coffee-lake-review-8700k-and-8400-initial-numbers/5#:~:text=The%20Core%20i7%2D8700K%20has,W%20is%20from%20the%20cores.
https://www.techpowerup.com/gpu-specs/radeon-pro-560x.c3282#:~:text=It%20features%201024%20shading%20units,rated%20at%2075%20W%20maximum.