There is no Mac made, including the iMac Pro, that can edit 4k H264 with total smoothness in FCPX (or Premiere) without proxies. I know because I'm a professional editor and I have both a top-spec 2017 iMac 27 and a 10-core Vega64 iMac Pro sitting on my desk. We use the GH5 a lot and the 10-bit codecs are even more difficult than the 8-bit ones.
For 8-bit 4k H264 in FCPX, a top-spec 2017 iMac 27 is *really* fast. It is about 2x faster than a top-spec 2015 model, likely due to the improved Kaby Lake Quick Sync. It's fast enough you can edit one stream of 8-bit 4k H264 without proxies. However all 8-bit H264 is not alike. But on all the Sony XAVC-S from our Sony A-series cameras and 8-bit 4k H264 material from our Panasonic DVX-200, Inspire 2 drone, GoPro Hero 5, etc, it handles it well. You still need proxies for multicam. Also FCPX is slow on the 4k 200 mbps All-I Panasonic codec, at least on 10.3.4. I haven't re-tested on 10.4 yet.
For H264 4k multicam you need proxies, but that's the case with almost any NLE on any hardware. For anybody who (1) Uses FCPX (2) Edits 4k H264 video and (3) Is on a 2016 or earlier Mac, the 2017 top-spec iMac 27 is a good deal. It's about 2x faster than a 12-core D700 Mac Pro on most H264 tasks, including ingest, proxy creation and export. It's way faster than any MacBook Pro 2016 and earlier. I haven't tested the 2017 MBP.
The iMac Pro doesn't have Quick Sync but apparently FCPX uses the dedicated video acceleration logic on the Vega GPU. It's much faster on H264 than a 2013 12-core D700 Mac Pro, but overall it's no faster than Quick Sync on the 2017 iMac. Therefore you are no closer to editing 10-bit 4k H264 without proxies. The iMac Pro is very quiet under high load -- almost like the trash can Mac Pro, so if acoustic noise is important that's a factor.
The Ninja Inferno will let you capture to ProRes which *can* be edited without proxies (inc'l multi-cam). However the files are about 6x the size of H264. On a ProRes workflow the iMac Pro is significantly faster than the 2017 iMac, but ProRes by itself improves editing performance on the iMac enough you might not need the iMac Pro.
The $4,000 deal sometimes available on the base-model iMac Pro is one option, also (if you can wait) evaluating the 2018 iMac update. If it has a 6-core i7-8700K that might be a good choice. If you need a machine today for improved FCPX editing performance on 4k H264, the 2017 iMac is a good choice and the least expensive option.