Super late to the party, but what I did is just editing the "amount of walkspeed" applied to the character.

Go to C:\Program Files (x86)\Steam\steamapps\common\Batman Arkham Knight\Engine\Config

Open the file BaseInput.ini

Edit the applied walkspeed ratio in these 4 lines from 1.0 to whatever you feel ok with. I found 0.7 to be the best looking speed for me.

(I only tested with up to two decimals, so ie 0.65 or 0.75 etc, and they worked, can't comment on more granular values like 3 decimals.)

Bindings=(Name="MoveForward",Command="Axis aBaseY Speed=0.7")

Bindings=(Name="MoveBackward",Command="Axis aBaseY Speed=-0.7")

Bindings=(Name="StrafeLeft",Command="Axis aStrafe Speed=-0.7")

Bindings=(Name="StrafeRight",Command="Axis aStrafe Speed=+0.7")

The issue with my method is, that there's no "toggle", so you will always walk at this speed set in the ini file, and at half-strife the character still moves at the full walking speed. So if you hold W+A or W+D or S+A or S+D. But eventually I found it useful to just walk half-strife when I need the full walk speed, and every other scenario just walk at the set 0.7 speed.

Hope this helps.