I think I'm seeing the most separation between QB ability on Heisman/Freshman, but that's the level that the INT slider didn't seem to have any affect, even at 95. It makes a difference on Heisman/Varsity, but the separation isn't there. And I see almost no separation at all on anything higher.
As much as Heisman/Varsity isn't working the way I initially expected, I still think it performs the closest to the ratings out of all the difficulty level combinations on the default slider settings.