Well I have a friend who is also on IRI, he once told me the difference between conventional locos and 3 phaser :-
According to him conventional locos get their powers in spurts due to the tap changers. Every jerked movement on the steering wheel like thing is actually like a higher level of power/torque or something like that. So whenever speed change has to be much depends on the skill of the LP otherwise passengers can have jerks in journeys.
3 phasers have joystick like throttle so that the power transfer in...
more... smoother and faster. Moreover they also seem to have some kind of a feedback system that makes the LP's job easier.
It would be better if you ask him, but he has not been online here now for a while, I will ask him more about it on facebook maybe.