Your eGPU is still being throttled by 20% with thunderbolt 3. Yes, your $1200 enclosure+decent GPU slapped to the $3000 brick still have the graphic performance of GTX 1080TI*80%= $640.
I am not impressed that the BlackMagic eGPU option is locked to AMD GPUs. Very weak, but then again Apple isn't supporting nVidia heavy iron. But the BM box isn't priced all that bad.
And yes from my perspective as well, TB3 is not fast enough to get true workstation class graphics.