...I'm wondering whether or not a top spec 13" MBP will be able to handle a 10 minute 4k timeline and still have a smooth editing experience (not fussed on render times, only the actual edit itself). Won't be using Premiere, only FCPX....I'm looking at a top spec 13" or a 15". Cheers
Editing H264 4k smoothly is very difficult for almost any hardware or software. You are using FCPX -- that's an advantage since it's so fast and uses Intel's Quick Sync for hardware-accelerated decode & encode.
However there are many H264 codecs and formats. Some are more compute-intensive than others. E.g, 4k H264 8-bit 4:2:0 from a Panasonic GH4 is OK, but the 10-bit 4:4:4 media from a GH5 is very slow. Yet the 4k 305 mbps H264 intra-frame content from a Canon XC10 or XC15 is very smooth and fast. Technically all those are variants of H264 or AVC.
A separate consideration is effects, which vary greatly in their CPU and GPU requirements. You could get decent timeline performance on the initial content but when you start adding effects, the edit performance will slow down.
In general the solution is defer adding compute-intensive effects until the timeline content is mostly locked down. E.g, don't try to stabilize clips or apply Neat Video noise reduction until the last steps.
For 4k editing you definitely want the fastest most recent computer you can afford. That said if you can use proxies (a built-in FCPX feature) almost any computer can edit 4k smoothly. You could edit multi-camera 4k on a 2013 MacBook Air using proxies. But you must generate those proxies at either import time or later, which takes some time and adds about 60% to the typical H264 camera media size.
I personally prefer the 15" MBP size because the screen is bigger and easier to see. I use a top-spec 2016 MBP 15" and it does OK on 4k H264 but I create proxies for anything besides a very short 5 min timeline. My 2017 and 2015 iMac 27s are both top-spec but the 2017 is significantly faster in FCPX on H264 content, probably because the Kaby Lake CPU has upgraded Quick Sync logic. If this is representative, then a 2017 MBP (which also uses Kaby Lake) might also considerably faster for this specific workflow than a 2016 model.