Honestly, I think that GO expansion/RER is going to be just as much of a crosstown line as the Crosstown itself. Most of the travel demand is to and from downtown so most of the Crosstown traffic is going to continue to feed into Line 1, and, later, the relief line and Kitchener, Lakeshore, and Stouffville GO lines. Most people won't be using it for trips across the uptown part of the city. The second reason is that even for cross-uptown trips, GO will be just as fast if not faster. The Crosstown is going to be permanently hobbled by the on-street portions that have trains stopping at red lights. With an average speed of 28 km/h, it would take about 41 minutes to go from Kennedy to Mount Dennis. A similar trip on a hypothetical through-running GO train would take about 45 minutes. With electrification and revamped platforms at Union, I could see that being reduced to less than 40.
So for a trip from one end of Eglinton to the other, your choices are going to be:
-An LRT that runs as a subway for half of the route but as a glorified streetcar for the other half, or
-An through running, electrified train that gets you there via downtown in less time for the same price.
I think RER really is going to be earthshaking for the 416.