Some games have an alternate version of the .exe to run the game for different DX versions. I have not heard of a game that requires DX11 yet, they always offer a DX9 version. Whether it's an in game option, or a different .exe, it should be there.
Benchmarks may be different. It wouldn't be a very good benchmark comparison if you were using a different DX version than what you are comparing to.
mostly for benchmarking ,, games like BF3 I can't change the directx version from the game menu and i don't know how
maybe DICE using the same method with BF3 as they did in BFBC2. with BFBC2 the game will automatically use the highest level of Direct X available to the card. there were work around to force the game running in dx10 or dx9 but i forgot how to do it. you can try googling the subject