Speed of GTX transceivers in Kintex 7 in FBG package?

Hi,

I have seen contradictory data about speed of GTX transceivers in Kintex 7 in FBG packages:
1. The datasheet ( http://www.xilinx.com/support/documentation/data_sheets/ds182_Kintex_7_Data_Sheet.pdf , page 54 ) states that the maximum speed is 6.6 Gb/s
2. The answer record http://www.xilinx.com/support/answers/50299.html states explicitly: " Limit the maximum line rate to 6.6 Gb/s if you need the core files
for an FBG package, even if the GUI allows for a higher line rate (For
Kintex-7 devices the maximum line rate is not dependent on package type
and can be up to 12.5 Gb/s for all packages while in Zynq it is limited
to 6.6 Gb/s for FBG packages and is 12.5 Gb/s for FFG packages)."

I haven't seen any warnings about rate limit in FB... packages in product selection guides like http://www.xilinx.com/publications/prod_mktg/7-series-product-selection-guide.pdf or http://www.xilinx.com/products/silicon-devices/fpga/kintex-7.html#producttable

I have tested a board, which was supposed to be assembled with  xc7k325t_ffg900 chip, but due to a mistake an  xc7k325t_fbg900 was used (chip was hidden under the heat sink, and in JTAG there is no difference between those chips)
The board worked perfectly at full speed with 10 Gbps links.

Does anybody has an information if this speed limit for FB package really exists, and what is its reason?

I can see two possible reasons:
1. Signal integrity. I know that there are differences between wire-bond
   and flip-chip packages (but AFAIK both FB and FF are flip-chip)
2. Heat dissipation. At 10 Gbps the link may dissipate more power 
   than at 6.6 Gbps. But does FB have higher thermal resistance than 
   the FF one?

The difference between the two above is significant.

In case 1 - if the board happened to work and Eye Diagram is good, it can be reliably used.

In case 2 - the board may work with link test only, but when we add additional processing blocks it may get overheated...

Thank you in advance,
Best regards,

Wojtek 
PS. This qustion was also asked on Xilinx forum:
http://forums.xilinx.com/t5/7-Series-FPGAs/Speed-of-GTX-transceivers-in-Kintex-7-in-FBG-package/m-p/638015#M12356

With best regards,

Wojtek