@payton it looks like you are operating the drivers with "fast" step pulse timing, which is the default for internal Trinamic drivers. This means the step pulses are only guaranteed to be long enough for the built in drivers to register a step.
External drivers generally need a bit of a longer step pulse. Now your motors are an odd design which use the stepper driver output to driver another driver. when a motor is connected the slight differences in step pulse length when stepping quickly makes no difference, but if you want the steps to be evenly spaced then try using the T parameter to extend the step pulse width (try T1).
You will observe however that if you go two fast the firmware will group step pulses together in batches which once again is a variable duty cycle however maybe your drivers wont mind that behaviour.
As far as step rate itself is concerned, with T1 you should be able to reach 300Khz, looking at your requirement (207 steps/mm at F16000 = 266.66 mm/s) of 55.3Khz step rate, this is very achievable.
