I have the top of the line 2016 5K iMac but I did 3TB Fusion drive instead of SSD. Every day I regret this decision. Don't sacrifice and get the fastest CPU instead go a line down and spend cash on the SSD. Final Cut Pro struggles just playing back a single camera angle on the iMac. I have a 4Ghz Core i7, 32GB of RAM machine that can't edit iPhone video .
I had a 2013 i7 with 3TB Fusion drive and still have all-SSD 2015 and 2017 i7 iMacs and a 10-core iMac Pro, all used for professional FCPX video editing. I tested the 2013 i7 iMac with 3TB FD vs the 2015 with 1TB SSD extensively -- boot time, app launching time, many FCPX encoding and performance metrics. There wasn't much difference.
There *is* a major difference in the 2015 vs 2017 i7 iMac in FCPX performance on 4k H264 material -- the 2017 model is about twice as fast, likely due to the improved Quick Sync logic in the Kaby Lake CPU. It is not dramatically faster on general workloads but editing H264 is a very common task.
Despite how fast the 2017 iMac is at FCPX, you still may need to create proxies for multi-cam H264 editing. Note that each H264 codec can have different performance characteristics, even if they are all 8-bit 4:2:0 and 100 megabit/sec. The XAVC-S material from the Sony Alpha series is somewhat sluggish, whereas similarly-encoded material from the Panasonic GH5 and DVX200 is smoother.
Mere "iPhone video" can be HEVC depending on the settings. HEVC material is vastly more compute-intensive than H264. Recent versions of FCPX on High Sierra or Mojave can use Quick Sync to accelerate this (for 8-bit, not 10-bit) but only for Skylake and newer CPUs:
https://www.anandtech.com/show/1061...six-notebook-skus-desktop-coming-in-january/3
If you are editing H264 or HEVC video that is a compute issue not an I/O issue unless you have transcoded to optimized ProRes media. In that case it can become I/O intensive but otherwise the data rates aren't that high.