You will need the speed to be set inside the spawned enemy in a [On Created] event that way each new instance overwrites the default speed value with the new one.
You will need a global "Speed" variable.
Make an event that increments the Speed variable every 5 seconds with action Variable Add: -100 (If your going right to left you will need a negative speed value).
Set your Enemy bullet speed to 0 in inspector and set its speed after creation using On-Created event: "On-Created - Set bullet speed -400+Speed.
Now just spawn away. I made a small .cpax that shows my work. Maybe this will help get you headed in right direction.