Hey MikeB, perhaps you should ask this in the PC hardware/software subforum in General Topics.
http://www.subsim.com/radioroom/forumdisplay.php?f=235
but as far as I know games that use DX11 can also use DX9 and 10.
I'm probably not one to ask but Id ont think you should have any problems. a lot of people having trouble with DX10-11 have found solutions in installing 9, so either way, I think there is plenty of cross-compatibility.