if it is possible to send and receive (say for example velocity commands) at more than 200 Hz

It should be possible, if you use a RT kernel, but I have never tried it.

Without a RT kernel the jitter is huge and you will not be able to control the drive properly. In my tests 100 Hz are fine on a non-RT kernel, 200 Hz is already getting unstable. Things get a little bit better, if you switch to the external BCM-based sync.

If not ? What is the reason ?

ROS and ros_control are multi-threaded and the thread scheduling is not reliable on a non-RT kernel.