A lot of that is beyond the scope of the project and for good reason, it is not that significant given how localised a lot of the environmental impacts of these developments are (dose makes the poison, and in this case it's all about proximity). Also, induced demand is a thing for airplanes just as it is for other modes of transportation, so the cheaper and more accessible you make flying the more people will opt to do so. You cannot make the assumption that the same number of people will fly either way.
In any case... in the medium term GO rail and the Pearson Rail Link should be electrified, which means that the amount of emissions released into the neighbourhood from people travelling to the airport by rail will be minimal. People who were going to walk or take transit to the island airport will be within walking distance of an electric rail connection to Pearson, so no extra cars on that equation.
People from the rest of the city heading to Pearson in cars will be heading to an airport in a very low-density area mostly through other low-density areas rather than through Toronto's most populated neighbourhoods.