Sorry, guiscard, on two systems here the GPU utilization is working as expected, on both 0.7.2 and 0.7.3. However, and again, I'm on Catalina, and further, I don't use FCPx or Compressor, but Resolve, Adobe, and Avid. Try running Geekbench and Cinebench to test out your GPU utilization. For a comparison, and using Radeon Boost, my Vega 64 runs a 73k GeekBench Metal score, and on the system with the Radeon VIIs, it can pull 100k scores. Start with that and if your scores are in the ballpark of mine (with RB), I'd start taking a closer look at your config.plist, and/or issues with BigSur and FCP/Compressor. i.e., you say you tried the two main SMBIOS flavors, but did you correctly configure/remove your GPU device properties for each?
Good luck,
J