I am working with 4 camera feeds, all Sony shot 50p in AVCHD, and a couple of go pros in 25p4k. I also have a couple of audio feeds. I film weddings so end up with around 20hrs of footage, but most of the work requiring proxies involved Multicam scenes where there are around 5 camera feeds and two audio for the entire duration of the scene, which can be anything from 20 minutes to an hour. There is no way that this part of the process will handle editing without proxy, but I have seen, is that even with proxies made, I get dropped frames when editing. My older 5k iMac handles the same project without dropping frames. This is not just a one off, even the second Pro I was sent has the same issues.I have thrown multiple streams of XAVC-S and XAVC-L at the iMP at "best quality" on FCPX with zero issues. The characterization that it needs proxies for smooth editing is just false.. unless you're talking about a 12" MacBook - which is where I need proxies. If you are working on the same kinds of footage I am, why do you need proxies?
Even an old 2013 Mac Pro didn't need proxies...
My experience does not match up with yours at all, except the 4K H.264 export times are faster on the 27" iMac. And both machines are far faster exporting H.264 4K than the old 2013 D700 8C Mac Pro.
Apple have now well and truly slammed the door shut now that it is absolutely crystal clear that there are issues with the IMP working with FCPX, and they just stick their heads in the sand. They have no complaints procedure and you can not even get past the local store manager as far as management is concerned. I have now lodged a case with Trading Standards, and would urge anyone having issues with this machine to do the same, as it should not be sold when there are problems with it working with Apples own software.