I'm interested in this too folks - sorry, I lack a little in CPU knowledge etc.
I've been desperate to upgrade a 2011 Mac Pro, for about 18 months.
I use my machine purely to edit video in FCPX and audio in Adobe Premiere.
Until now, I've shot everything in 1080p, and have been surprised at how long I have had out of this Mac Pro - about 8 years.
It doesn't do well with 4k video - and you need to use proxy files.
Would this new 2019 iMac, with the i9 CPU upgrade handle raw 4k video?
And is the RAM user-upgradeable?
Really appreciate the help - and sorry again if these seem like basic questions.
Ross