i'm having the same problem... i made the "autoexec.cfg" folder, opened it with notepad, and put in mat_dxlevel 80 but it didnt do anything
my console still reads my mat_dxlevel as 70.
EDIT: I am a complete fucktard, i put in "dx_matlevel 80" instead of "mat_dxlevel 80"..... problem fixed, for me atleast.
only thing i can say that may be wrong is that when you save the "autoexec.cfg" file make sure it's NOT "autoexec.cfg.txt" cuz that is what mine tried to do the first time