You said:

"just 400kHz"? Show me a stepper drive which can input more than that.
And tell me how safe is to put MHz signals into cables in noisy environments.
Doing that is megalomanic idiotism.
Furthermore the OP needs not much more than 100kHz frequency.

The DYN 4 uses MHz signaling in noisy environments successfully (as do many many drives)
and has a 500 KHz maximum step/dir input rate (And many servo drives take much higher)
So your original statement was basically just plain wrong...