The maximum line rate for the GTX Transceivers in the Virtex-6 CXT family have been updated to 3.75Gb/s and version 1.4 of the Virtex-6 FPGA GTX Transceiver Wizard does not yet support the faster rates in the -1 speed grade. This answer record describes how to work around this limitation.
To work around this constraint, it is possible to temporarily change theCORE Generatorproject such that it targets a -2 speedgrade. This can be done from the Project->Project Options menu. Once the Wizard outputs have been generated, the project speedgrade can be changed back for any other IP that need to be generated.