Is a 600+Hz R/C Servo/ESC refresh rate possible?
The standard R/C hobby servo pulse width range of 1~2ms gives a maximum repetition rate of just under 500Hz, but if a non-standard range is used then it can be higher. High-speed servos designed for helicopter tail rotors are often designed accept 410~1140us (centering at 760us instead of 1.5ms) which theoretically could be sent at up to 877Hz.  

The manufacturer's specifications for the XRotor-20A say this:- 


  1.5 Compatible with various flight-control systems and supports a signal frequency of up to 621Hz (Note: all throttle signals over
  500Hz are non-standard signals)


Although servo pulses are a type of PWM, they encode the servo position into a specific pulse width rather than an on/off ratio. the servo or ESC detects the rising and trailing edge of the pulse and measures the time between them, so the repetition frequency has no bearing on the decoded position. Having the line 'just pulled high' is invalid because there would be no pulse to measure!