Don't you think that you are overplaying your backplane power capabilities here?
Tom's Hardware tested the load at each power input and
at full load, a Radeon VII can spike up to 322W with ~298W sustained (~30W from PCIe slot, ~142W from the first PCIe power connector and ~126W from the second). Even with a Powerlink as a load balancer, MP5,1 can only provide ~240W from both PCI AUX A and B before the backplane monitoring circuit shutdowns the PSU.
Maybe I am; all I can say is I've been using this for a few weeks now and had not one shutdown issue. No problems during Furmark, Luxmark or the usual Heaven and Valley benches. The card is down-volted to 970 mV, which surely helps, and monitoring has never seen more than 8 A drawn from each boost slot, so ~192 W from those in total.
In fact, I'll grab Luxmark and run it again now just to see.
That's pretty typical of what happens; power through the aux connectors pegs at 7.99 A and through the PCIe connector at 3.22 A, so that's around 230 W, which is in the ballpark of what the PowerPlay settings should be. I have it set at 200 W @ 120% so
shouldn't see more than 240 W draw in total.
I'll attach the PowerPlay kext I use, which is 1890 MHz @ 970 mV, 1111 MHz HBM2 and 200 W (120%) limit.
Oh, if you look carefully at iStat you'll see there's no reading from the Aux A slot because I carelessly knocked the monitoring chip off the logic board some time ago. But the cables are physically ganged together even before they go into the PowerLink, so they'll be evenly balanced.