As far as I know, only the cream-coloured "Robin Trower" Full-Drive has a different circuit than the rest of the FD-2's. I believe that the difference between the Full-Drive (original) and the FD-2 is a different op-amp chip (if I remember from reading on their site a while back).
The "Flat Mids" is the new midrange setting on the latest version of the FD. In the "Vintage" setting, it's the same as my FD-2.
I personally love the idea of having the comp-cut on a switch instead of the pull pot, because I've stepped on the comp-cut enough times to just keep it engaged when I play now ... you get a slightly brighter, more agressive tone when the compressor is out of the signal path.
I've grown so used to the smoothness of the compression circuit, tho, that I just leave it on all the time.