The PWM output frequency is settable via the RC_SPEED parameter, but 20 kHz seems more like the frequency for ESC output updates, not their inputs. I don’t expect that will work (it seems way too high), but you can try it and see what happens.
Note that a 20kHz frequency has a 50µs period, so you’ll need to do additional modifications to ArduSub to avoid the current hardcoded 1500µs pulse-durations used for the neutral/stopped signal (as I mentioned initially).