As for a GO station at Summerhill, two points:
1. Most people in Toronto do not work downtown. Many people getting off the GO line at this station would be going to destinations north of Summerhill.
2. The purpose of putting GO service on the CP line through Summerhill is for crosstown trips that avoid downtown, as this would be a line that links Milton, Meadowvale, Cooksville, Kipling, Summerhill, Agincourt and Malvern. The purpose of this line is really to get people off the 401, not to get people downtown. Although it is true that some people from Scarborough would use this line to get downtown, people coming from Milton/Mississauga would have a choice: use the Milton line that goes to Union Station or use the crosstown line that goes to Summerhill and Malvern. My guess is that we would see alternating trains from Milton/Mississauga going to Summerhill/Malvern and Union Station.