Odd news on the P5020 evaluation board. I inquired about the default SerDes configuration and got this info.

Factory SerDes configuration for the P5020DS board is 0x36:
PCIe1 x2
PCIe3 x2
SGMII EC1
SGMII EC2
SGMII EC3
SGMII EC4
Debug x2
XAUI 10GEC
SATA1
SATA2

Of course it is possible to modify the RCW to use another SerDes configuration.

So as shipped, the PCIe lanes are relatively limited. Luckily we already know that we can reconfigure for one X8 PCIe or three X4 PCIe configurations.