You don't state the voltage your using to feed the servos. But if you read the specs, the max rpm is 4000 rpm, and the Ke is 18.75 V/(should read 1000rpm).
To achieve 4000 rpm, you need 18.75*4000/1000 or 78.75 volts which is very close to the limit of the gecko.
Obtaining ~2300 rpm would indicate a voltage of ~43V unless I have missed my math.
Tuning could also be a problem if you have the correct voltage.
Just my thoughts.