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While unit moves, individual models can’t leave their ranks and try to move as one entity by keeping unit formation.
The only exception is Retreat maneuver. When unit starts to flee from enemy its models continue to move together but as a mob in chaotic fashion to simulate turmoil and panic.

tests notes

Leader M and I values doesn't affect unit RS

Cavalry unit M value is overridden by its mount type M value as soon as its armor is set to mounted type.  Mount M value is most likely hardcoded as there is no way to change it in WHedit. 

Mount type Horse M value equals to M7 of infantry unit (confirmed by tests)

Real speed formula

RS formula depends on unit's M and Ini characteristics as well as some modifier which transforms Movement tabletop metrics to Dark Omen ones. The logic is the higher the M and the I values the faster is a unit.For instance, unit

RS = a*M - b*n1 I *n2- c

Wherewhere:
RS - unit real speed
M - unit Movement
I - unit Initiative
n - modifier value >0 to <1
a - M modifier 
b - I modifier
c - modifier

DO precise formula of unit real speed (RS) is unknown at the moment.is RS = 0.4*M - 0.1*I - 0.2

Acceleration/deceleration formula 

depends on M and I
precise formula is unknown at the moment.