The problem is the folks who worked to make these things "scale friendly" are no longer with Lionel. I wonder who made the decision that the original U33C's ran too fast. I surely didn't get that feeling with my EL unit.
I think the only change made was to the worm and worm gear.
It's only a theory, but it might be possible to pull the hirail wheels off the worm gear axle and replace them with the scale wheels from the 6408142625 conversion wheel set. Only the worm gear wheelsets should need to be changed.
A lot of trouble that shouldn't be necessary, but a possible work-around.
Rusty