I'm a video editor who uses a top-spec 2015 MBP and a top-spec 2013 iMac 27 for FCP X video editing. They both work OK on both HD and 4k, although effects processing on 4k can take a lot longer. I don't recollect any "horror stories about 5k iMacs choking on FCP X".
Contrary to popular belief, most time-consuming video editing tasks are CPU-limited, not GPU or I/O limited. Anybody can see this themselves by editing video and monitoring system activity with iStat Menus or other tools. FCP X can effectively edit most camera native formats without transcoding to optimized media. This greatly reduces I/O burden. If you edit optimized ProRes material the I/O load will be higher, and of course 4k is higher still. So in increasing order, the I/O load of various material is camera native HD (typically H.264), 4k camera native, optimized ProRes HD, then finally optimized ProRes 4k. For any of those multiple video streams (multicam, etc) will also increase the I/O load.
The GPU is important but less so than the CPU. Time-consuming editing tasks such as transcoding and rendering of certain effects are CPU-bound and are not dramatically accelerated no matter how fast the GPU.
You want good GPU and I/O capability, but CPU is the number one thing -- the more cores the better. Of course on an iMac or MBP you don't have much choice in this area -- I'd suggest getting the fastest available 4Ghz 5k Imac with the M295X GPU, a 256 or 512GB SSD and a Thunderbolt drive array.
I will probably get the next upgraded 5k iMac and am not worried about it "choking on FCP X". It will be significantly faster than my 2013 top-spec iMac.
