I play NCAA extensively, and just bought Madden. So I am going to get some sliders down here soon. In NCAA I was forced to move some sliders up. This was because alot of College players have low awareness rating and other ratings, there aren't too many elite players in the 95 to 99 range. Thety weren't performing as they should at 50. So even cranking up the AI sliders didn't kill the gap between the players. The Elite - Mark Ingram's, etc, still felt elite.
But with my limited Madden experience, it appears that moving the sliders up even just a bit can make alot of running backs and corners Elite. It used to be back in the day, I was fairly confident that every 5 ticks you moved a slider, = 1 point. So if you moved Running Back Ability form 50 to 75, a 92 speed RB would now be 97 speed. The elite players were capped at 100 so the gap narrowed. Conversely if you moved down slider to 25, the 92 back is now 87 and the 97 is now 92, actually even more elite by % of variance.
I am not even sure what Attribute the Break tackle slider affects now, but I guess it would be speed as well, probably everything but ball carrying which is controlled by the fumble slider.