so we don't have to keep going back and forth, With the current FMCOMMS1 board you can not recover the exact bits that yo utransmit, on the recieve path.
If you are asking us why we used 16 bit DAC and 14 bit ADC, this decision was made internal to ADI. You can desgn your own board based on our schematic, layout and gerbers and replace the ADC if you like.
As far as the VGA, I'm not sure I understand what you are asking?
Charly