To route a rail line towards Pearson, the costs would be pretty enormous but well worth it I think. You get a double benefit by being able to create a full HSR corridor that both Go and VIA trains could use. Connecting the Airport straight into the Georgetown line would have huge benefits, and the Airport's basically a necessity for good HSR service.
I would personally love to see the massively bloated blue 22 proposal canned and a Newark AirTrain type service between Woodbine and Renforth put in its place. If people can check bags anywhere on the system, and stations are designed so users only have to walk across a platform to transfer, then I could see such a service being sufficient. Such a system would also be able to serve parking areas and terminal areas much more effectively with more local service than a heavy rail corridor. Keeping the high speed station and alignment on the Georgetown line would allow for better through service, as well as provide better regional and limited service for those coming from the west and not going to Pearson. Baggage handling service could easily be added to the stops at Woodbine and Renforth to further streamline the process, as well as as ticket agents for planes and trains.