Increasing the Due's I2C clock speed

I am using a HaydonKerk Stepper motor which can theoretically move 30 mm in 6.6 seconds (under 500 lbs of load). However, with the Adafruit_MotorShield I am only able to get 30 mm in 16 seconds. The documentation for the motor shield suggests that one way to boost the speed of a stepper is to change the TWI_FREQ constant (located in twi.h) from 100000L to 400000L. However, when I did this, there was no difference in speed of the stepper. Is it possible that I need to do something differently for the DUE?

All you need to do is: Wire.setClock(400000L);

I run one of my I2C channels at 2MHz.

Regards,
Ray L.

I tried 400000L and 20000000L and there was no difference in stepper speed.

Any other thoughts? In the documentation they specify changing the value in twi.h manually. Is it possible that I must do this for the compilation of the libraries to recognize the new i2c clock rate?

NOTE: I know that the stepper is in fact capable of higher speeds, I tested it on the phidgets stepper motor controller (based on libphidgets2) and was able to get significantly faster velocity.