Kazu,
Yes, I think you are correct.
Basically the payload size of C66x PCIe outbound transfer (C66x masters (CPU/EDMA) initiate the PCIe write/read transfer to remote device) is 128 bytes.
And the payload size of C66x PCIe inbound transfer (C66x PCIe master port completes the write/read transfer initiated from remote device) is 256 bytes.