I'm not sure what you are referring to regarding the powermod. I tried googling but the only thing that came up was due to the Radeon VII being so power hungry, you would need an external power supply to drive it properly.
While you are experimenting with Twinmotion, try putting the quality on Ultra across the board and then adding tons of grass painted onto sculpted terrain, and then adding lots of trees and plants, in addition to the Archicad model you sync.
Powermod would be the Pixlas mod or external power supply. I too use dual mini 6-pin to 8-pin AND dual SATA power to 6-pin right now with this Vega56. I am planning to do some kind of power mod though, with one of my machines at least.
Red Dragon Vega56 with low bios mode test:
I stole some time today for short experiments with twinmotion. I did not paint any grass on terrains, but I tried some water and about 300 live people in a swimming hall. Twinmotion model consists of 12.2 million polygons and 1.1 GB of textures.
Twinmotion played my model at around 25 FPS constantly at ultimate settings. I know it's not as fast as some or most games do play, but it's easily a workable solution for architectural vizualisation aid. Renders are fast, in seconds or tens of seconds. I think materials need some tweaking, they differ quite a lot from cinerender output in ArchiCAD.
Strangely it played at 25 FPS with almost every setting, low, medium, high or whatever, so it could be the bottleneck is processor or something else, not the Vega56. I noticed Twinmotion did not ever use more than 400% of CPU resources, which means 1/6 of total CPU power in my twelve core setup (12 cores + hyperthreading makes it a total of 2400% max according to apple system performance tool).
From screenshot you can read some performance and resources statistics info; GPU 95% and CPU 55% does not reflect what I said above though, so I don't know why it behaves like it does. I recon it's not a completed product yet, so we'll see what happens later on. I tested few hours without a crash - not bad. Next time something more demanding. And planning to compare this with ProRender in C4D too.
I would think your planned 1st choice of a card (as I read it above) WX9100 would achieve at least this, and probably better.
I am still considering a VII myself. They can be bought under 700€ at times, as new.
edit. without water effects and without any moving people in viewport, I get 102 FPS out of TW.