in any case, I do agree with you that Supermicro needs to step up for their X299 boards with the review and samples. otoh, I'm still not sure why they need to produce consumer line products when their main business is server/workstation with Xeon/Epyc
As a baseline, this is the temperature derating graph for IR3550 (the prior generation before IR3555)
VCCIN is 1.8V , so derate by another 25% (1.25x loss multiplier).
Elmor claimed eight IR3555 can do 340W on Apex.
If you want 400W with 1.8V VCCIN you're looking at ~223A of current. With an 8 phase board that's about 28A per phase. (32A with a 7 phase)
If you want 600W with 1.8V VCCIN you're looking at ~333A of current. With an 8 phase board that's about 42A per phase (48A per phase with a 7 phase). Per the temperature derating graph you'd have to be below about 60°C ambient/board temp if you want to leave a margin for error.
I don't want to give you wrong information, but that is what we know thus far.