Heathrow is such a poor comparison for Pearson it's not even funny.
1. The Greater London Area has no fewer than 4 international airports of significance. Heathrow, Gatwick, Stansted, and Luton. A city centre airport London City. And a minor airport Southend. An analogue in the GTA would be Pearson, Hamilton, a currently un built Pickering, and the Island airport. And that still leaves another international airport and a minor airport
2. Together those 6 airports comprise 8 runways for the Greater London region. Hamilton, Pearson, and the Island comprise 9 runways. 2 of which are alternate runways in Pearson's 15/33 runways and 2 of which are secondary runways mainly used for light aircrafts (1 each at Hamilton and the Island). So best case scenario is that the GTA operates with a maximum of 5 runways at any one time. Less that 50% of the runway capacity in London
3. There has been a loonng discussion in England on adding another runway somewhere in Greater London. Each of Heathrow, Gatwick and Stansted have put their plans in for adding another runway, and there has been the grand plan for the airport on the Thames river. So it's hardly genuine to suggest that London is getting by just fine with it's current situation. Pearson on the other had has said that it is not the runway system which is the limiting factor but the terminals and gate capacity.
You may as well say that the GTAA should look at Atlanta or O'hare airports as models with 4+ parallel runways.