Capacity will not be a proble. This is a high capacity line and remember that by the time it opens, RER will be going strong so you won't get as many people from Scar taking the subway in the first place.
What I'm interested in is what happens to Queen when the line is done. Do they still intend to run the Queen streetcar? Are they going to make Queen a pedestrian only zone or is the city going to continue to place pedestrians at the back of the bus and just ;et it stay the same as it is now? I don't recall this being discussed but it's an important issue when it comes to the effect the OL will have on the city's urban development and realm.
In this case, I would go with retaining the streetcar service, while tweaking some of the routes. For example, today both #501 and #502 cross Yonge. Once the OL opens, #501 can continue serving downtown while #502 can be shortened and just connect the Kingston Road loop to the closest OL station.
OL by itself has a wide stop spacing, and does not stay on Queen for much of length. It cannot replace the streetcar; if the streetcar was removed then a bus route would be needed instead.