the issue causing the most significant delays for Perth isn't runway capacity, it's airspace capacity
For departures yes. Departure rates are throttled during peaks because the TWR can fire them faster than we can deal with. By sorting the departures into a more efficient order leaving the aprons the departure capacity could be raised tomorrow.
For arrivals it is a question of runway capacity because we can process/stream aircraft faster than the TWR can land them. For example 03 ILS's arrival rate is about 17/hour. At that rate approach will only have about 4 aircraft on frequency at time.
Without sorting the departures, parallel runways would probably make departures worse because often the aircraft on the left runway would need to turn right and vice versa.