Ok, so to balance out my disappointing results with PhotoScan, here's my impressions of playing around with the Unreal Engine editor:
HOLY CRAP IT'S FAST NOW!!!
The difference in performance between my 10-core, Vega64 iMP, and my Late 2015 i7 iMac is so astounding that I don't even need to run exact benchmarks to see the difference.
1) Initial post-install compilation of the 3000 or so built-in shaders finished in just 2 minutes. Normally I'd be waiting around for 10+ minutes as they ticked down one-by-one.
2) Changing a master material (and thus all of its instances too) in a project I'd worked on before, recompiled in 7 seconds. With the results appearing in the viewport in about half a second. I'm used to this taking 20-30 seconds on the older iMac for the recompile, and 5-10 seconds for the materials to actually update in the preview window. This will speed up my material editing exponentially.
3) The biggest gain is in rebuilding the scene's light maps. Went from 3-4 minutes, to ~25 seconds. With the CPU being fully utilized for a good chunk of that.
Obviously Unreal Editor is very well optimized for multi-core systems.