It's 11 minutes after the GO train, that the shuttle arrives at York. With 4 minutes for the GO Train to travel to Downsview Park, that gives 7 minutes to walk into the subway station, down all the way to the platform, wait for a subway train that only comes (in AM peak) every 5 minutes, take the subway 2 stops (about 4 minutes), and come all the way up out of the station. There's no way the extra GO ride and subway is faster. If you just miss a subway train, you could still be at Downsview Park getting on your suwbay train, when the shuttle bus is already finished.