Yes exactly. I don't know I suppose it depends on the dwell time differences between using Union as a terminal station vs through station. Combined with use of Summerhill station (the great rail hype of regional transit in the GTA) could that not relieve pressure off of Union by up to, say, 25%?
I really don't know what the difference would be, and whether that difference would be enough to eliminate the capacity crunch. In all of my maps though, the only line that uses Union as a terminus is the Milton line (it's the odd man out, so to speak).
As for Summerhill, it would certainly be of value to a Midtown GO line, but I don't think it would be a good idea to pour too many people into there, because of the Yonge capacity crunch. The only way I would put a lot of pressure on Summerhill is if it became the terminus of a Bay St subway (aka a Yonge Express line). But that's a pipe dream for sure.
For what it's worth, here's the quick map I did of what I'm talking about: