1) If you want to keep big plays at a minimum, use 100 threshold. If you want to see Chris Johnson and Felix Jones rush for 400 yards and 6 td's; and DeSean Jackson have 12 catches for 500yds use 0 threshold LOL!
2) The higher you go up on sliders(aka all-madden) the lower your threshold should be because with the sliders jacked up, the speed boost is already included. With the threshold set at 100 along with jacked up sliders, you can kiss the fly pattern and any big plays goodbye.
... the sliders have no effect on player speed. Not in the terms you're referring to anyway.