The PLX chips are the PCi-E bridge chips that give extra PCI-E lanes on the board. I don't know if I would call them switchers.
The electrical assignment per slot in the WS is automatic. I don't remember any settings in the bios that allows manual control other than PCI-E Gen 3,2, or 1.
If you plan on dual GPU's with a raid and Blackmagic card then yes you would need the WS board.
Eric
ADK