I believe that in truth there is no definite line or link between what renderer the studio chooses, and the final look of the game.
The choise of Maddox Games to only have active support for DX10+ level hardware and software is a logical one, as they intend for IL2:CoD to endure far into the future, and this is also the trend within the entire gaming industry. If you look at
Steam's hardware survey, which is a quite decent representation of the average gamers PC, one can see that DX10+ software/hardware combination are now in domination, and growing rapidly. with the recent release of DX11, studios would have to make certain that things work on DX9, 10 and 11, and as such it makes sense to start dropping WinXP and DX9 entirely to conserve effort. The only thing still keeping DX9 (of which the latest release is now just short of 7 years old) in circulation is that multplatform games require DX9-level coding for the X360 and PS3. Some PC games, such as Just Cause 2, requires Vista/Win7 + DX10 to start at all.
In regards to what DX9, 10, 10.1 and 11 are capable of, it mostly revolves around improved minimum hardware requirements to be regarded as "DX10-compatible", streamlined pipelines, improved performance and a few added functions such as hardware per-object motion blur on DX10 and Tesselation on DX11. Read
here for some more info.
The final look of the game is very much in the hands of the studios in charge. Take a look at Far Cry 2 and Crysis as an example. Crysis is actually primarily a DX9 game, even on maximum graphical settings, as DX10 support was added into the game core rather late in the production. Several mods have been released which enable most of the supposed DX10-level functions even on DX9 hardware, with the exeption of some motion-blur and some high quality atmospherical shaders, and with reduced performance due to the optimized pipelines of DX10 hardware. As such when you look at images like these keep in mind that they are (arbitrarily estimated) 90% DX9 and 10% DX10.
Image 1. Image 2. Image 3. One could summarize it as such, that a mediocre studio can use the best resources and achieve nothing, while a good studio uses less and achieves leagues. (I am in no way implying that Maddox's creations are mediocre, though a bit inoptimized, which I have no doubt will be remedied promptly. I am using the saying in a industry-spanning perspective)