The IBERT design is not using all of the MGTs in the design, yet the power consumption of the device is much larger than expected.
What is causing the extra power consumption from the Virtex-4 device?
In the Virtex-4, all of the MGTs on your device will be powered and active regardless of how many MGTs you are using in your IBERT design. This will cause your device to draw larger than expected power since all MGTs will be active. For a true indicator of the power consumed in your design the estimator should be used on the actual design not the IBERT test design.
This condition does not apply to the Virtex-5 GTPs and GTXs.