I have the 10G-T version of that board/SOC. You're sure the CPU is your limiting factor for Plex, right, and you know more clocks will fix the problem for sure? I ask because I run an Emby server (same use as Plex) on mine along with a bunch of Dockers and really don't have any problems with it. It handles multiple transcodes, can zap through things like Roku thumnail generation quickly (this uses ffmpeg and will max your cores if you let it) and so on. And I'm able to keep it quiet and cool.
As you've probably gathered from the thread so far you're going to pay a fairly pretty penny to upgrade what you have now along with some compromises. There's no straight path to "exactly what I have now, just faster and under $1500". And you may not need it. What problems are you seeing now? There may be other ways to alleviate them.
Things may change in a few months as the Xeon-E becomes more widely distributed, etc.