You are correct. If you leave it at 50 it's "realistic breakaway speed". If you put it to 0, it actually amplifies the differences between players - so good players will become VERY good and have ridiculous breakaways more often.
I feel like everyone making sliders this year has failed to figure this out (along with other things
) - which is why I write off most slidersets with 0 threshold. Charter has left his All-Madden sliders at 50 threshold and also has CPU accuracy below 10 - there's a lot of reasons for this.
My guess is the ridiculous CPU completions is coming from the "high" CPU pass accuracy (anything over 10). Remember people, AVG NFL pass completion is below 60%. If you are getting 90%, that's so far off from real that you might as well be playing a Madden challenge mini-game rather than a game of simulation football.
Just because a sliderset is "hard", doesn't mean it's "good".