I've fixed the mouse input issue.

Mouse input for this game is borked on default settings. The main reason for this whole issue is that the Controller Settings also affect Mouse Settings!

These settings make the input curve of the controller 'flat' and that in turn makes the mouse input far more linear.

Set Universal Soldier Aim/ADS Field of View "ON" for Mouse and Controller before proceeding.

Set Controller Zoom Sensitivity Coefficient: 0
Set Mouse Zoom Sensitivity Coefficient: 0
Set Controller Input Curve to "Linear" and Input Accel Curve "BFBC2" which is also flat and has no accel.
Set Controller Input Deadzone to 0.
Set Controller Soldier Aim Sensitivity to 0.
All Aim Assist settings to 0.

My mouse settings/hardware:

FOV 80
Mouse DPI: 400 - Steel Series Aerox 3 Wireless
Soldier Mouse Aim Sens: 8

I'll monitor this thread to help anyone who needs it. This game is so much more fun once this issue is dealt with!