Personally, I'd prefer to have North and South hubs to serve Pearson, connected by a People Mover (likely Mark III ICTS). The North Hub would be Malton GO station (HSR, Via, RER, FWLRT), and the South Hub would be Renforth Gateway (Transitway, ECLRT). Still provides easy access to the airport via the People Mover, but the transit hubs are in locations where they're useful to even non-Pearson riders.
This setup would also justify UPX, since it would be the only direct-to-Pearson service. The other connection options would still be good though.