Q: How low can a Model Y go on these compared to normal lowering springs?

On the Model Y these let the rear go lower than the factory spring arm allows without forcing you onto an extremely short spring with harsh rates and little travel. That was a specific design goal: more drop while keeping usable spring travel and ride quality. UP set racing lap records with both the Model Y and Model 3 running these arms, the kind of setup you can bring to a TeslaCorsa track day, then kept daily driving on them.