Give the niagara component the following uproperty and it should work
UPROPERTY(VisibleAnywhere, meta = (AllowPrivateAccess = true))
class UNiagaraComponent* NiagaraComponent;
Here it is in a custom c++ class 
Definition in c++ class header
And the custom user params in the particle system
And if I remove the float override I set in c++ then yes it does show the original 800 value bp => c++ class
