Yeah, autosubs are funky because they are related to the fatigue slider (and the fatigue slider has a significant impact on the gameplay). In general, increasing the autosub "In" value (set to 80 by default) results in a quicker player locomotion (i.e. sliding). Lowering it can produce better footplanting - but at the cost of momentum - which can lead to some bad interactions along the line and tackles.
Back to threshold....
Threshold is a really key value, even a 1 point difference can greatly begin affecting the overall gameplay balance. This isn't inherently a good thing nor a bad thing but it sets the base environment for the game. While threshold does change the difference in speed/accel separation, higher threshold does not necessarily equal fewer big plays - it depends greatly on how the other sliders are set. Big plays are often about timing (i.e. where are blockers set up relative to the runner) and pursuit angles. Threshold will impact both of those things because you've changed the relative speeds in which players are moving.
I find the interaction of the threshold slider fascinating. In some ways, I'd like it to have no impact on the game and let player ratings determine a play where speed and acceleration are factors, but again, this may not necessarily be realistic. For example, Richard Sherman has never been one of the quicker corners in football, but continues to be one of the most effective due to his ability to read the play and get where he needs to be despite his relative lack of speed.
I dont know that the AI in Madden can replicate that innate intelligence that someone like Sherman displays. To equate to our conundrum, having the threshold slider set compensates for failings within the game mechanics to allow defensive backs to make tackles that would otherwise not be made, thus keeping the game 'realistic'.
But it's very interesting how many slider gurus vary on their perception of the speed threshold slider and where it should be set. Not just a variance in setting but how this fundamentally shapes the rest of the sliders to build around it.