Now it comes to my mind when Counter-Strike source was released (long ago), in official tournaments, dx7 mode was banned, because it allowed you to have better visibility through fog effects, compared to dx8 and dx9 modes.
Latter on, i think Valve removed completely the option to run the game in dx7 mode (though i'm not sure about this last thing).
So it's more or less the same case, a performance-oriented setting was banned because of the unfair advantage it granted.