DoN. Nichols writes:

If the Gecko 320 is the one which drives servo motors (but
pretends that they are steppers), then it does nothing with the tach
feedback from the servo motor, and uses the encoder to tell which
"stepper motor step" it is at. Feeding it several steps very quickly
will increment a counter which is compared to another counter run from
the encoder which produces a voltage to drive the servo motor in the
intended direction until the count from the encoder catches up. At
certain speeds, you have the servo motor moving at pulsing speeds,
just like the stepper that it is pretending to be, and thus have the
resonance problem again.

This is speculative fantasy. Gecko controlled servos do not behave this
way. PID digital feedback yields smooth motion.

Note that the Gecko drive, with a servo motor, will still be
producing the steps, because it does not try for a steady-state speed.

More silliness. That's not how PID digital controls work.

The step/direction signals are simply a method of *communication*
between PC and controller. The motion does not exhibit stepping.

Your efficiency notion is also wrong. Geckos use PWM MOSFETs which are
very efficient. Heatsinks are hardly necessary for many high-power
applications.