>> The x8+x8 bifurcation of this x16 controller is fixed in silicon
>> and thus cannot be changed.

> Not even using a PLX switch?

I think using a PCIe switch on the x8 slot could work as this has nothing to do with the bifurcation of the POWER9's PCIe controller.

> the SAS controller is optional, the board appears to be available without it.

I guess the lanes dedicated to this SAS controller are routed to the footprint no matter if populated or not.