All done, no hiccups whatsoever after getting it started.
Note to Vega users: set HBCC before launching Blender (or presumably any application that will be using VRAM) as it locked up and I had to end and restart. Not a big deal, but worth remembering I think.
HBCC set to ~11GB, RX Vega 56 flashed to 64 Air BIOS, Win10, 32GB@2933MHz, Driver 17.9.3, Wattman set to Balanced, GPU pegged at or above 1616MHz for all 36ish minutes. Now I’m off to the gym, hope this helps.
Edit: oh yeah, temp lived at 75°C ±1° with fan at about 2000RPM. I should’ve had HWiNFO open but forgot and didn’t want to launch sensors during render.
I’m not sure how HBCC assigns VRAM, as Blender was “using” about 9.5GB, but Radeon Settings (while open of course) sat at ~120MB. I believe that it links the RAM assignment to the application using VRAM though because the usage by Blender dropped to under 1GB immediately as the render ended.