No techie, but from what I remember the main change was notably due to Ackerman and the throw of the steering arm. The horizontal arm (X-ray T4 for your example) meant the steering throw was pushed and pulled in a linear sweep and that apparently translated into smoother control, especially through long sweeping corners.
Electronic space saving due to servo case orientation helped for motor gearing when a huge juicy pinion was needed for stock classes.
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