You're reading too little into the difference in lenses.
The*ist DS and K100D are using the SAME lens, (and the same sensor) so it's really only the processing differences which will show up.
and.. MTF comparisons.. how do you think that's going to be done? Generally it'll just be the same film (sensor!) and different lens, right? (and. d'oh. look what we have here.)
Next - you do know that the pentax 50 1.7 is regarded as "sharper" than the 50 1.4 right? It's just that the 50 1.4 is supposed to be excellent (across all makes) and that might be the only thing causing the difference.
I'm just saying that the possibility should be considered.