Well I can actually quantify with somewhat certainty the performance hit.
My "test" was load up The dead man walking chapter where you walk out of "the wall street building" or w/e and look down at the soldiers down below and open fire.
Before ( dx9 ) : All settings max + custom config + 2x SGSSAA + 2x MSAA + d3doverrider the fps hovers between 55 and 60 depending on amount of action on screen. 60 when "calm"
After : All settings ultra ( max ) + high res pack + high motion blur +d3dovr., no custom config, no externally forced AA, fps hovers around 55 and 60 depending on amount of action on screen. 60 when "calm"
So in other words, dx11 goodies + high res texture ~ 2xSGSSAA ( in terms of performance hit ), which is gosh darn significant. So I understand people saying the hit feels big, because it is.