The GTH Transceiver User's Guide notes that the TX Port Width is limited too 16 bits. This Answer Record describes the other port widths that are available.
Solution
In the GTH Transceiver, when the 8b/10b encoder is enabled, the allowed port widths are 16, 32 and 64 bits.
This will be corrected in a future release of the GTH Transceiver User's Guide, UG371.