That picture is pretty awesome to take a look at the VRMs.
I'd interpret the design like that:
In detail I'd expect the powIRstages to be IR3555 parts, the IR35201 running in 5+X mode and five more IR3599 on the backside.
Besides MSI the other manufacturers have their product pages online by now:ASRock Fatil1ty X399 Professional GamingASRock X399 Taichi
-> 8x IR3555, most likely driven by one IR35201 running in 8+0 mode for CPU VCC.Gigabyte X399 Aorus Gaming 7
Again 8 phases equipped with powIRstages, driven by a IR controller. Seems to only have two Soc phases.
Originally Posted by AlphaC Gigabyte
supposedly will be using 8 phase IR3553 per Gamers Nexus
On the picture on their website you can see that the powIRstages used aren't IR3553 packages.