IBERT and v1.8 of the Virtex-6 FPGA GTH Transceiver Wizard are currently setting the SLICE_NOISE_CTRL_x attributes to incorrect values as follows:
SLICE_NOISE_CTRL_0_LANE01 => 5007h
SLICE_NOISE_CTRL_2_LANE01 => EFFFh
SLICE_NOISE_CTRL_2_LANE23 => EFFFh
Updating these attributes to the correct values (as shown below) can provide a potential upside to the RX margin on some heavily utilized designs, but the majority of the designs will see no impact.
However, it is still recommended to update these attributes to the correct settings and updating them should not result in any negative impact.
This has been updated in the Virtex-6 GTH Transceiver wizard and IBERT in the ISE 13.2 release.
SLICE_NOISE_CTRL_0_LANE01 => 0000h
SLICE_NOISE_CTRL_2_LANE01 => 7FFFh
SLICE_NOISE_CTRL_2_LANE23 => 7FFFh