Success indeed! Thanks to this thread.
Flawless update from a Vega 64 to an MSI RX 6800XT trio, Big Sur 11.4. OC 0.7.0, iMac20,2.
Just added agdpmod=pikera in the boot-args. More than doubled my Geekbench Metal score, from 60750 with the Vega 64 to 150950 with the 6800's. Really? Hard to...