0.4ms must be 0.4 sec.5.4 ms inductive time constant and 0.4 ms inertia time constant
1msec speed servo loop time and 100 msec position servo loop time enough for your application.
But if you want to draw circle by 2 pid controller and motors your position servo loop time must decrease (1msec)
At the past I have same question therfore I used 100 micro sec for the position servo loop time.
If I want to design new digital pid controller I select 1msec servo update time for position servo. Because I read following talented article.
This is same type question.
1msec very long time for 2MHZ encoder pulses this is true but PID need error.
Also this article really good.