The Virtex-6 SX475T -1 speed grade device has an incorrect DRC check in 14.6 GTX IBERT which limits the GTX line rate to 4 Gb/s. Similarly, the -2/-3 speed grade DRC allows a rate of up to 10.31 Gb/s.
The rate limits shown in the Virtex-6 data sheet (DS152) are correct. The -1 speed grade GTX should be limited to 5 Gb/s and the -2/-3 speed grade GTX should be limited to 6.6 Gb/s.
To work around the DRC limiting the -1 speed grade line rate, you should generate a -2 speed grade NGC using the CORE Generator tool and include it in your design:
Following these steps will get the design past the erroneous DRC check while still including the proper routing and timing for the -1 speed grade device.