This can be tweaked in an easier way. Just use commands (AFAIR):
stereo ps=1.0
Where 1.0 is substituted by factor you want to multiply by
You can invoke the “Execute console command” BP node somewhere during the start of the application (e.g. BeginPlay of PlayerController).
I’ve written article that has some info about VR Head model in UE4 for UE4 blog: Creating Escape Velocity for Samsung Gear VR: A Postmortem - Unreal Engine
Maybe you’ll find it helpful