For POWERSTEP01, in the STSPIN Family Eval Tool GUI max speed is 15609.741 Steps/sec Is this also the max speed for STEP mode (where I provide the step input)

The value of 15610 step/s is the maximum speed allowed by the integrated motion engine (for example when the logic processing the SPI commands).

Using the Step-Clock mode (refer to stepclock command in the powerSTEP01 datasheet at page 76) you can overcome this limit.

In Step-clock mode the motor motion is defined by the step-clock signal applied to the STCK pin. At each step-clock rising edge, the motor is moved one microstep in the programmed direction and absolute position is consequently updated

Maximum step-clock frequency is about 1.5 MHz.